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UNLV Retrospective Theses & Dissertations
1-1-2008
Pedogenesis of vesicular horizons in disturbed and undisturbed Pedogenesis of vesicular horizons in disturbed and undisturbed
soils soils
Maureen L Yonovitz University of Nevada, Las Vegas
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Repository Citation Repository Citation Yonovitz, Maureen L, "Pedogenesis of vesicular horizons in disturbed and undisturbed soils" (2008). UNLV Retrospective Theses & Dissertations. 2331. http://dx.doi.org/10.25669/5pee-rmj7
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PEDOGENESIS OF VESICULAR HORIZONS IN DISTURBED AND
UNDISTURBED SOILS
by
Maureen L. Yonovitz
Bachelor of Arts in Geology Hope College
2005
A thesis submitted in partial fulfillment of the requirements for the
Master of Science Degree in Geoscience Department of Geoscience
College of Sciences
Graduate College University of Nevada, Las Vegas
May 2008
UMI Number: 1456380
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Thesis ApprovalThe Graduate College University of Nevada, Las Vegas
November 16 ,2007
The Thesis prepared by
Maureen L. Yonovitz
Entitled
Pedogenesis of Vesicular Horizons in Disturbed and Undisturbed Soils
is approved in partial fulfillment of the requirements for the degree of
Master of Science in Geoscience
/r,
Exam ination Committee M ember
J &Examination Cqm mittee M ember
f U V ____
Graduate College Faculty Representative
1
Examination Committee Chair
Dean o f the Graduate College
11
ABSTRACT
Pedogenesis of Vesicular Horizons in Disturbed and Undisturbed Soils
by
Maureen L. Yonovitz
Dr. Patrick Drohan, Examination Committee Chair Assistant Professor of Pedology Pennsylvania State University
Increasing desertification and anthropogenic soil disturbance is of growing
concern to restoration ecologists in the Southwestern United States. This study examines
how a common soil horizon of arid lands, the vesicular horizon, which is characterized by
its discontinuous pores, responds to disturbance. The vesicular horizon is a near surface
soil horizon typically composed of fine grained windblown material found beneath a
desert pavement. Disturbance to the vesicular horizon may result in a change in pore
morphology that will alter water movement through the horizon, affecting the ecology of
the region. This research examines differences in pore morphology (area, perimeter,
length and width), particle size, pH, calcium carbonate content (CUE) and conductivity
(EC) between disturbed and undisturbed soils. Results indicate no significant difference
in vesicle pore size with disturbance. Analyses of pore shape do indicate differences in
types of pores formed; the percentage of pores with vesicular shape decrease while
interstitial (irregular, between grains) shaped pores increase with disturbance. An
increase in interstitial pores may result in more facilitated water movement to lower
111
horizons than there would be in a soil with discontinuous vesicular pores, which could
affect the vegetation growing in these areas. The re-formation of vesicular horizon
porosity in a disturbed soil may be related to its prior (undisturbed) physical and chemical
characteristics.
IV
TABLE OF CONTENTS
ABSTRACT............................................................................................................................. iii
LIST OF TABLES..................................................................................................................vii
LIST OF FIGURES...............................................................................................................viii
ACKNOWLEDGMENTS...................................................................................................... ix
CHAPTER 1 INTRODUCTION......................................................................................... 1Formation of Vesicular Horizons................................................................................ 2Formation Mechanisms ........................................................................................6Role of Desert Pavement in Vesicular Horizon Formation...................................... 9Vesicular Horizon Stability........................................................................................ 11Purpose of Study.........................................................................................................11Hypotheses...................................................................................................................14
CHAPTER 2 METHODOLOGY......................................................................................20Study Area Location.................................................................................................. 20Field Methodology..................................................................................................... 24Laboratory Methodology...........................................................................................24
CHAPTER 3 RESULTS.................................................................................................... 32Sample Site Descriptions...........................................................................................32Analyses Across Sites................................................................................................ 33Analyses Between Disturbed and Undisturbed Soils............................................. 44
CHAPTER 4 DISCUSSION.............................................................................................. 49Interpretation...............................................................................................................49Pore Morphology........................................................................................................50Physical and Chemical Analyses and Pore Morphology........................................ 64Physical and Chemical Analyses.............................................................................. 65
CHAPTER 5 CONCLUSIONS.........................................................................................71
REFERENCES....................................................................................................................... 72
APPENDIX 1: DATA TABLES............................................................................................76
APPENDIX II: BACKGROUND ON ARID LANDS SOIL PROCESSES IN THE MOJAVE DESERT................................................................................................................ 93
BACKGROUND APPENDIX REFERENCES.................................................................115
VITA.......................................................................................................................................121
VI
LIST OF TABLES
Table 1 Table 2 Table 3a
3bTable 4a
4bTable 5 a
5b Table 6 Table 7 Table 8
Table 9 Table 10 Table 11
Field site locations................................................................................................ 21Description of field sites.......................................................................................34
Comparisons of pH, EC, and CCE mean values across sites............................36
Comparisons of total particle size, sand and silt size fractions across sites .... 37
Comparisons of pore measurements across sites................................................40Comparisons of vesicles versus vughs based on L W* measurements.......... 41Comparisons of vesicles versus vughs based on visual observations............. 42Percent difference in vesicles between L W* measurements and visualobservations...........................................................................................................43pH, EC, and CCE mean values between disturbed and undisturbed samples. 46Particle size data between disturbed and undisturbed samples......................... 47Pore measurements between disturbed and undisturbed samples....................48
VII
LIST OF FIGURES
Figure 1 SEM image of a vesicular horizon...................................................................... 3Figure 2 Proposed dust accumulation process (Anderson et al., 2002)...........................5Figure 3 Vesicle development model proposed by Evenari et al. (1974)....................... 7Figure 4 Restoration ecology of a closed road................................................................. 13Figure 5 Line drawing identifying major micromorphological features seen in a
photomicrograph of the platy bottom section of a vesicular ped (Anderson etal., 2002).............................................................................................................. 18
Figure 6 Study site locations............................................................................................. 21Figure 7 Field Photos of Callville Wash, Cottonwood Cove and Airport Flat.............22Figure 8 Rainfall simulator............................................................................................... 26Figure 9 Examples of L W'^ ratio measurements based on pore orientation................27Figure 10 Histogram showing distribution of length to width ratios of all pores
measured...............................................................................................................28Figure 11 Examples of a vesicle, vugh and interstitial pore............................................ 29Figure 12 Callville Wash UD pores with 1/w ratios of 1.1 and 1.7,1 to r ....................... 56Figure 13 Callville Wash UD pore with 1/w ratio of 1.7 (vugh)...................................... 56Figure 14 Airport Flat D pore, classified as interstitial....................................................57Figure 15 Airport Flat UD pore with L-W* ratio of 1.2 (classified as vesicle).............57Figure 16 Pore size data for disturbed and undisturbed sites based on a Mann-Whitney
test..........................................................................................................................60Figure 17 Convoluted-edged or interstitial pores. Airport Flat Disturbed...................... 62Figure 18 Smooth edged ovoid pores. Airport Flat Undisturbed..................................... 62Figure 19 Irregularly shaped, closed pore, Callville Wash Undisturbed........................ 63Figure 20 Total silt and silt fractions based on a Kruskal-Wallis Comparisons test 66Figure 21a Kruskal-Wallis total sand and sand fraction charts.......................................... 67
21b Kruskal-Wallis total sand and sand fraction charts continued......................... 68
Vlll
ACKNOWLEDGEMENTS
First and foremost, I would like to thank my advisor. Dr. Patrick Drohan, for his
ability to be in two places at one time. Even though he lived across the country, he was
always available to help me with anything 1 needed. Thanks also go to Dr. Rob Fairhurst,
for his assistance with the SEM, and to committee members Dr. Matt Laehniet, for all his
helpful comments. Dr. Brian Hedlund, for taking time out of his extremely busy schedule
to sign forms and deal with scheduling issues so that 1 could graduate on time, and Dr.
Michael Wells, for his last minute assistance. 1 would also like to thank Dr. Clay Crow
for making sure 1 followed laboratory procedures and did not blow anything up. 1 would
also like to thank the members of the soils group, particularly Josh Boxell, Colin Robins,
and Mike Howell, for their assistance with lab analyses. 1 am very appreciative of Dr.
Jeff Herrick, Justin Van Zee, and all the members of the Jornada Project, ARS, for
providing me with field data and photos, as well as the impetus for this project. Thanks
also go to the UNLV Geoscience Department Bemada E. French Scholarship and the
Graduate and Professional Student Association Travel Grant for providing me with
funding for my research. 1 would also like to acknowledge my family and my friends in
the Geoscience Department, particularly Tonia Arriola, for all of their support and for
helping me to increase not only my knowledge of geology, but also of life in general.
Finally, 1 appreciate the opportunity that the UNLV Geoscience Department has given
me to further my education in Soil Science and prepare me for my future career.
IX
CHAPTER 1
INTRODUCTION
Anthropogenic soil disturbance in arid climates is of growing eoneem to
restoration ecologists in the Southwestern United States due to decreased stability
following disturbance (Vollmer et al., 1976; Bolling and Walker, 2000; Harrison et al.,
2002). Frequently disturbed is the surface soil vesicular horizon.
The vesicular horizon is a near surface soil horizon typically several centimeters
or less but generally no more than 20 cm in depth (MeFadden et al., 1998), which derives
its name from the tiny pores (or vesicles) found in the horizon (Springer, 1958; Evenari et
al., 1974); vesicles are transitory and non-continuous and spherical or oval in shape
(Figueira and Stoops, 1983; MeFadden et al., 1998) (Figure 1). While the horizon is not
officially recognized in the U.S. Department of Agriculture, NRCS Soil Taxonomy,
many working in arid ecosystems use the notation Av to identify the horizon in field
descriptions and note the horizon as a separate soil horizon when describing soils.
In laboratory experiments, vesicles have been found to re-form very quickly after
disturbance and repeated wet/dry cycles; re-formation of vesicles usually occurs after
approximately 5 wet/dry cycles, with the formation of a continuous vesicular horizon
after about 20-30 wet/dry cycles (Springer, 1958; Figueira and Stoops, 1983). Recent
research in the Southern Nevada area with specific reference to this study has led to
hypotheses that in the field vesicle re-formation can occur within a season, or four to six
months, after a rain event (Herriek et al.. Buck, Drohan, personal communication). This
is contradictory to restoration ecology studies of disturbed arid soils, which hypothesize
that recovery of such soils from disturbance takes a century or more; in many cases
recovery is believed to never occur at all (Bainbridge et al., 1995; Webb, 2002). A key
question then is what does recovery entail and does a soil need exhibit all pre-disturbance
characteristics to still be functionally similar following disturbance. In this study we
quantitatively and qualitatively define differences in pore morphology in disturbed and
undisturbed soils in order to assess potential functional soil changes due to disturbance of
the surface horizon. This study is the first to document the morphology of pores that
have been re-formed following disturbance in the field.
Formation of Vesicular Horizons
The vesicular horizon is defined as being composed of particles in the silt to clay fraction
(MeFadden et al., 1986, 1987, 1998; Evenari et al., 1974; Quade, 2001; Anderson et al.,
2002), which are believed to have been transported and deposited by eolian processes
(Evenari et al., 1974; Wells et al., 1985). The vesicular horizon material is often overlain
by a dense silt cap or crust that helps to stabilize the more loosely consolidated material
below (Evenari et al., 1974). Wind tunnel experiments have shown that crusts overlying
the vesicular horizon can become progressively weaker with wind erosion over time
(Langston and McKenna Neuman, 2004). Protection of the vesicular horizon from wind
disturbance may come from a rough, stony surface, such as a desert pavement, which also
helps to stabilize the soil (MeFadden et al., 1986, 1987; Quade, 2001). Studies of desert
pavements in the Cima Volcanic Field have shown a similar composition in bedrock and
Figure 1. SEM image of vesicles in a vesicular horizon from Callville Wash.
pavement material (MeFadden et al., 1986, 1987, 1998; Anderson et al., 2002).
However, differences in chemical composition that often occur between the soil and rock
material at a site leads to the conclusion that the bedrock is not the sole parent material
for the vesicular horizon and that other fines comprising the horizon must be derived
from elsewhere (Anderson et al., 2002). It is hypothesized that this material is brought in
as dust, which then is trapped beneath the desert pavement and accumulates upward,
causing pavement uplift (Anderson et al., 2002; MeFadden et al., 1986, 1987, 1998). The
uplift process proposed by Anderson et al. (2002) occurs in three phases: (1) initial
stages of eolian accumulation, possibly influenced by adhesion and surface tension forces
between water and sediment; (2) continued eolian accumulation of clays and CaCO]
allow for vesicle formation through shrink/swell processes in the clays and vesicle
cementation by carbonate (shrink/swell then enhances stone lifting); (3) continued
shrink/swell creates structure and allows for more accumulation of eolian material
(Figure 2). However, whether pavement development and surface stabilization or the
accumulation and formation of the vesicular horizon occurs first is unknown.
Based on their study of pavements in the Cima Volcanic Field, MeFadden et al.
(1987) suggest colluviation of basaltic clasts on top of eolian sediment filled depressions
as a method of pavement formation; in this case, the vesicular horizon material was
proposed to have been in place before the pavement.
The vesicular horizons present in the Mojave Desert today are hypothesized to be
Quaternary in age, formed during the last glacial/interglacial shift from the late
Pleistocene to early Holocene, during which time a climate change caused the desiccation
of pluvial lakes and a large increase in dust deposition (Quade, 2001 ; MeFadden et al..
, . PHASE r r ,lam inar crust, v esk ies
co lum nar and platy peds
CWGEl single grain
lam inar crust
PHASE 2 laminar crust
vesklK
I - eolian sediment Q } - cdum nar ped boundary [ [ ] - basalt bedro k or dasts
I -vesicular soil C j-p la ty structure -B horizon |< r ! sed movement direction
Adhesion and .urAke tensltm Active Processes (Idealized)
Shrlnk/sweH (and salt crystallization)
TTanslocition m d m a tr ic lb rc e s
Figure 2. Proposed dust accumulation process (Anderson et al., 2002).
1986,1998). Exact ages are not as certain, as vesicular horizons are difficult to
accurately date using radiocarbon methods because their accumulation is often
continuous through the Quaternary (Anderson et al., 2002). Implications for relative
dating may, however, be linked with these soils’ associated pavements, since their
formations are believed to be genetically related (Quade 2001). Pavement relative ages
for the Mojave Desert are in accordance with the climate shift as a result of the Last
Glacial Maximum (LGM) in the late Pleistocene (Quade, 2001). According to Quade
(2001), the cooler climate of the last glacial period caused vegetation zones to move
down to lower elevations where climate conditions were drier; observations of current
pavements have led to the conclusion that they do not develop in well vegetated areas
(where moister would be more prevalent) (Quade, 2001). Therefore, in the Mojave
Desert, only pavements below an elevation of -400 m could have persisted through the
LGM and be Pleistocene in age as these were the only areas that were low enough to
remain relatively vegetation free. Elevations between 400 and 1900 m could have only
had strong pavement development during interglacial periods and therefore any
pavements present here must be Holocene in age. Elevations above -1900 m do not have
desert pavements (Quade, 2001).
Formation Mechanisms
Though the exact characteristics that lead to vesicle formation have yet to be
determined, vesicles within the vesicular horizon are thought to be caused by a
combination of factors/processes having to do with the movement of air and water
through the soil. One such process occurs as rain water moves downward into the soil.
pavementcbso
Raîn infiltrâtes Soil Expansion of heated air from rain
Air trapped beneath cruA or pavement Vesicle Development
Figure 3. Vesicle development model proposed by Evenari et al. (1974).
As rainwater infiltrates the soil and percolates downward, it displaces air upward with
vesicles developing as a result of trapped air (Evenari et al., 1974). Vesicles are
hypothesized to develop as a result of the expansion of heated air in the soil by the
overlying surface crust or desert pavement (following surface drying and exposure to
solar radiation) (Springer, 1958; Evenari et al., 1974) (Figure 3). Soil crusts are also
believed to promote vesicular horizon development by protecting the underlying horizon
from damage by external forces or by adding to pore cementing materials via leaching,
translocation and transformation of ions (Singer and Shainberg, 2004). Soil crusts are
thin, dry layers at the soil surface that, in some cases, prevent water from entering the soil
(Singer and Shainberg, 2004). Soil crusts can be formed by either mineral or biological
processes. In mineral crusts, cementation of secondary carbonates and/or salts increases
soil compressive strength at the surface (Jergman, 1988) resulting in a crust. Biological
soil crusts (or cryptobiotic crusts) are made up of cyanobacteria, lichens and mosses and
are often associated with vesicular horizons, and thus may affect (or be affected by)
vesicular horizon development (Belnap et al., 2004). A relationship may also exist
between the genesis of pedogenic carbonate and microbial processes, contributing to the
material cementing the crust. Monger et al. (1991) present an experiment in which they
blew heated air across the tops of soil columns, creating a vapor-pressure gradient that
allowed for the release and upward flow of Ca^ ̂in solution. Calcite only formed in soils
that contained microorganisms. These results indicated that the presence of calcite in
desert soils cannot be solely attributed to inorganic precipitation (Monger et al., 1991).
However, Evenari et al. (1974) and MeFadden et al. (1998) conclude that while microbes
may have some influence on calcite morphology and microbiological crusts can
contribute to vesicle development, the causes of calcite precipitation and vesicle
formation are mainly inorganic.
Role of Desert Pavement in Vesicular Horizon Formation
The association between the vesicular horizon and desert pavement has been of
interest due to the effect of desert pavement on surface stability, infiltration, and eolian
dust accumulation. Quade (2001) suggested that desert pavement and the vesicular
horizon are formed by related processes but that these processes are of some debate. Two
processes are proposed that attempt to explain pavement formation above the vesicular
horizon; 1) pavements are generated at the surface while the vesicular horizon rises
underneath with continued dust accumulation (MeFadden et al., 1987); 2) pavements
develop above vesicular horizons due to the shrink/swell capacity of clays in the
vesicular horizon (Figueira and Stoops, 1983; Anderson et al., 2002; Springer, 1958;
Buck et al., 2002).
Although pavements may be important in trapping the eolian dust that later forms
the vesicular horizon (MeFadden et al., 1998), they have also been shown to decrease the
infiltration of water into the soil (Abrahams and Parsons, 1991). Because water is
thought to be a key factor in the vesicle development process (Evenari et al., 1974),
factors affecting infiltration could therefore influence where vesicles are best developed.
In an attempt to quantify the amount of infiltration that is able to move through desert
pavement, Abrahams and Parsons (1991) found a negative correlation between the
amount of stone cover and water infiltration when compared to infiltration on adjacent
soils with vegetative cover, meaning infiltration is lower with a higher amount of stone
cover. This may be due to desert pavement having less plant cover as compared to non
desert pavement soils (Musick, 1975). Musick (1975) also observed that desert pavement
soils are usually saline-sodic or sodic; exchangeable sodium causes dispersion of the soil
particles, slowing down infiltration, decreasing soil moisture and inhibiting plant growth.
Young et al. (2004), using a tension infiltrometer to measure unsaturated water flow in
the vesicular horizon, found that older soils with desert pavements and higher silt and
clay contents hold water longer in the upper part of the soil profile; however, pavement
clasts were removed for this experiment so that bare soil was exposed for infiltrometer
measures (Young et al., 2004).
While the underlying bedrock in an area may be similar in mineralogical
composition to the desert pavement found at the surface (Anderson et al., 2002), the
mineralogy deriving the vesicular horizon may or may not be different. Within the
Mojave Desert, several different parent materials have been found to contribute to the
formation of vesicular horizons, including basalt, limestone and mixed alluvium. Basalt
parent materials that support vesicular horizons and pavements have been extensively
studied at the Cima Volcanic Field in the Mojave Desert of California (MeFadden et al.,
1986, 1987; Anderson et al., 2002; Wood et al., 2005). These studies have shown that
although a limited quantity of the vesicular horizon may have been derived directly from
the basalt, a difference in chemical composition between the basalt and vesicular horizon
indicate the horizon’s formation is strongly influenced by eolian processes (MeFadden et
al., 1987). Supporting observations of eolian contributions of vesicular horizon
mineralogy in the occurrence of carbonate collars underneath basalt clasts; had the basalt
10
been the sole contributor to the underlying soil’s formation, the presence of carbonate
clasts would be highly unlikely (MeFadden et al., 1998).
Vesicular Horizon Stability
The degree of vesicle stability within the vesicular horizon remains undetermined.
The fine grained material that the vesicles form in leads to the assumption that they are
extremely fragile (Springer, 1958). Even though the vesicles may be transitory and
unstable (Figueira and Stoops, 1983), clay and silt layers that form around the inner edges
of the vesicles may provide some stability as well as be evidence that vesicles can remain
intact (Sullivan and Koppi, 1991; Springer, 1958). CaCOs may also provide vesicle
stability through the cementation of grains that surround the vesicle (Evenari et al., 1974;
Anderson et al., 2002).
Purpose of Study
Vesicular horizons play an important role in arid ecosystems affecting surface
stability and water movement into the soil. Although vesicular horizons are made up of
fragile eolian silt and clay materials, once weakly cemented, they can provide stability to
arid land surfaces by acting as a protective cover for the underlying soil (MeFadden et al.,
1998). Yet if disturbed, the fine grained materials that make up this horizon could be re-
released into the air as dust, causing both ecological and health problems; dust in arid
environments that are highly populated can lead to several chronic illnesses (USE?A
2003). In addition, vesicular horizon porosity is unique in that pores are not
interconnected (Figueira and Stoops, 1983; MeFadden et al., 1998). This characteristic
11
leads to reduced surface infiltration (Young et al., 2004) and thus less water movement to
lower soil horizons. Disturbances to the soil surface therefore alter infiltration if porosity
is changed by disturbance. The purpose of this study is to examine the chemical
composition and pore morphology of disturbed and undisturbed vesicular surface
horizons to determine the characteristics that lead to vesicle formation and how this
changes with disturbance. From an ecological standpoint, this research is important in
helping to determine how one aspect of soil function (porosity of the vesicular surface
horizon) changes following disturbance. Given that it is now estimated that >30% of the
terrestrial land surface is affected by desertification (Bowker et al., 2006; Eswaran et al.,
2001) (including much of the western U.S. (Belnap, 1995)) affecting over 44 million
people worldwide (Eswaran et al., 2001), this potential change in soil function affecting
water movement can affect whether a soil recovers in terms of ecological function and
thus overall ecosystem stability. This research quantitatively and qualitatively defines
differences in pore morphology in disturbed and undisturbed soils in order to assess
potential functional soil changes due to disturbance of the surface horizon, providing
additional knowledge to land managers for restoring disturbed soils in arid ecosystems.
Some areas, such as that shown in Figure 4, have undergone or are currently
undergoing restoration practices in order to help these environments return to their
natural state. Specifically, with this study, 1 seek to:
(1) further research on the pedogenesis of disturbed vesicular horizons, which will
bring new insights to the processes by which the vesicular horizon forms in soils with
respect to the conditions needed for their formation. To do this, 1 examined the particle
12
Figure 4. Increased disturbance of arid lands by humans has become a concern of restoration ecologists. The above image is of a road that has been closed (behind posts) and “re-planted” with dead plants in order to deter off road vehicle use in the area as well as act as a dust trap and stabilizer for the soil surface (Joshua Tree NP, CA).
13
size, calcium carbonate content, pH and conductivity of the vesicular horizon in order to
determine its source material as well as whether any of the above chemical or physical
characteristics of the soil are contributing to vesicle formation and stability. This
information can then be applied to a variety of field restoration ecology efforts on
disturbed lands.
(2) further research on the effect disturbances have on vesicular horizon crust
development and soil function (Belnap et al., 2004; Johansen, 1993). When
experimentally disturbed, the vesicular horizon has been found to re-form relatively
quickly (after about 5 wet/dry cycles in the laboratory) following a simulated rain event,
implying these horizons may re-form more quickly and easily than commonly thought
(Springer, 1958; Figueira and Stoops, 1983). One question that arises from this
observation is how the vesicles can form so fast. The vesicular horizons in my study
come from three differently aged surfaces in the Mojave Desert near the Las Vegas area
(Airport Flat, Cottonwood Cove and Callville Wash), composed of three different soils.
As part of a study being conducted by the U.S. Agricultural Research Service, all soils
were subjected to rainfall simulator irrigation for 30 minutes each at an average intensity
of approximately 107 mm/hr and excavation to 40 cm.
Hypotheses
1. The influence of the vesicular horizon on soil moisture and infiltration is an
important consideration for land managers practicing restoration ecology on
disturbed desert soils. A goal of restoration ecology is to restore a disturbed area
to a similar functional form; thus a lack of understanding of the effect of
14
disturbance on the vesicular horizon limits the effectiveness of restoration efforts.
Therefore an objective of this research is to compare the genesis of vesicular
horizons in disturbed vs. undisturbed soils.
a. I hypothesize that landscape surfaces with greater original stability (higher
percent clast cover, thicker vesicular horizon, and higher chlorophyll
content in the biological crust) will re-form better-expressed vesicular
horizons (thicker horizon; more, larger and more rounded pores) following
a disturbance.
b. I hypothesize that while pore size increases, total pore space will decrease
with age in vesicular horizons in undisturbed soils; in disturbed soils pore
space will always be higher than in the undisturbed counterpart.
Rationale for Hvpothesis (a)
To complete this objective, I will examine how pore morphology changes
in terms of pore shape, size, area and perimeter between these sites in order to
determine how the pores themselves differ in the new vesicular soil following
a disturbance. To test this, I will measure pore morphology (area, perimeter,
and the length to width ratio).
Sites with greater vesicular horizon stability can be identified as those
with a higher percent clast cover, a thicker vesicular horizon, and higher
chlorophyll content in the biological crust, which indicates the presence of a
cryptobiotic crust. Past research has shown that better expressed vesicular
horizons are thicker (MeFadden et al., 1986, 1987; Anderson et al., 2002) and
have a higher percent of silt and/or clay (MeFadden et al., 1986, 1987). In
15
this study, the sample site with the best expressed vesicular horizon, based on
these conditions, was Cottonwood Cove; samples taken from the disturbed
tracks at this site should therefore re-form with the best porosity out of the
disturbed samples from all three sites.
Rationale for Hvpothesis (bl
Two potential mechanisms could explain how total pore space would
decrease with age in undisturbed soils. (1) If clay or carbonate is able to
accumulate around inner vesicle edges following wetting and drying events
(Sullivan and Koppi, 1991), thus maintaining vesicle stability (Evenari et al.,
1974; Anderson et al., 2002), the vesicle will not be destroyed and re-formed
but will instead continue to accumulate more layers until it has been
completely filled. (2) If vesicle stability is not maintained during rain events,
vesicles will begin to coalesce upon repeated wetting and drying cycles
(Figueira and Stoops, 1983), thus forming fewer but larger pores, and
decreasing overall total pore space in the process of coalescence. Coalescence
may have occurred if size, shape and continuity are different across the
different aged sites (i.e. older soils with vesicular horizons will have less but
larger vesicles than younger soils due to more time for
deformation/reformation cycles).
If pore development may be recognized based on vesicle shape, my
objective is to determine how this changes with a disturbance. Anderson et al.
(2002) found that, with increased vesicular horizon development, pores
become more continuous near ped bottoms, forming a platy structure that may
16
eventually lead to increased horizonation and development within the soil
profile. 1 hypothesize that this is caused by wetting fronts that create a low
permeability clay layer, leading to air/water interfaces over time that perch
water making more continuous, less rounded pores (Figure 5). If this is the
case, assuming that vesicular structure has been completely destroyed in a
disturbed soil, newly re-formed vesicles should be less continuous, smaller
and more rounded in disturbed than in undisturbed samples.
2. A second objective of this research is to investigate the role of precipitates, such
as CaCOs, and particle size in vesicular horizon formation following a disturbance
as compared to undisturbed soils. This is important because differences in
porosity, i.e. disconnected vs. more well-connected pores, could lead to
differences in infiltration affecting available water to many desert plant species.
a. 1 hypothesize that the degree of expression of a vesicular horizon
(determined in this study by pore size - area, perimeter, length and width,
and roundness (1/w ratio)) in a disturbed soil is dependent on particle size
while that of an undisturbed soil is due to particle size and cementation by
CaCOg. Initial pore formation is most likely due to particle size until
CaCO] accumulation has been established enough to cause cementation of
grains. Cementation would be disrupted in a disturbed soil. A lower
CaCOs content in a disturbed sample would therefore indicate vesicle re
formation is more dependent on particle size.
b. 1 hypothesize that the extent of pore development can be recognized based
on vesicle shape, and more rounded, well-developed vesicles will
17
Figure 5. Line drawing identifying major micromorphological features seen in a photomicrograph of the platy bottom section of a vesicular ped (Anderson et al., 2002).
18
form in soils with higher silt and clay content while more elongated, less
developed vesicles will form in sandy soils. This is because finer textured soil
has greater adhesive forces between particles due to smaller spaces between
pores, thus increasing aggregate stability (Hillel, 1980). In sandier materials,
CaCOs would likely be necessary for the cementation of the coarser grains.
Rationale
Vesicular horizons form in a range of sand, silt and clay fractions
(McFadden et al., 1986, 1987, 1998; Evenari et al., 1974; Quade, 2001; Anderson
et al., 2002; Sullivan and Koppi, 1991; Figueira and Stoops, 1983; Springer,
1958), and are generally more prominent in finer textured materials, such as silt
and fine sand, than coarse sand (Evenari et al., 1974). Springer (1958) observed
that, upon wetting, particles first changed their orientation to hold water at the
surface before infiltration and then became more closely packed together as air
moved through the pores. These changes in particle behavior led him to conclude
that vesicular formation depends on specific textures and structures in the soil.
Textural differences have also been observed to occur within an individual ped,
with coarser (sand sized) material on the outer edges and increasing clay content
toward the ped center and bottom (Anderson et al., 2002). CaCOs concentration
has also been found to increase toward the centers and bottoms of vesicular soil
peds (Anderson et al., 2002). Coarser textured soils, such as a sandy loam or
loamy sand, would require CaCOs cementation of grains in order to maintain
vesicle stability.
19
CHAPTER 2
METHODOLOGY
Study Area Location
The study area consists of three different site locations within the Lake Mead
National Recreation Area, near Las Vegas, NV (Figure 6). Based on soil morphology
and geomorphology, sample sites were ranked in age youngest to oldest: Callville Wash,
Cottonwood Cove and Airport Flat (Figure 7). Callville Wash soils are part of the
Gypwash Series; they are formed in limestone alluvium on fan remnants (NRCS
Gypwash Series description; See Appendix). Cottonwood Cove soils are part of the
Huevi series, which are formed in mixed alluvium on summits and side slopes of fan
remnants (NRCS Huevi Series description). Airport Flat soils are part of the Cheme
Series, formed in mixed alluvium on fan remnants (NRCS Cheme Series description).
Airport Flat is considered to be the oldest of the three surfaces because it is located at the
highest topographic setting and has not been subject to recent dowm cutting events, which
the other two sites have. In addition. Airport Flat soils are formed on a silica pan
(duripan). Cottonwood Cove is on a lower terrace than Airport Flat. Callville Wash soils
are located directly within a wash, which has been subject to the most frequent dowm
cutting events, therefore qualifying Callville Wash as the youngest surface. Further
description of the locations of the three sites is provided in Table 1.
20
LegendSample S ites
I I Lake Mead NRA
I I Clark County, NV
Sample Site Key AF = Airport Flat CA = Callville W ash CO = Cottonwood Cove
#Las Vegas
0 10 20 80Kilometers
Figure 6. Study sites in the Lake Mead National Recreation Area east of Las Vegas in southern NV USA. Codes in legend apply to sample sites (triangles) seen as photos in Figure 7.
Table 1. Field site locations (UTM Zone U N , NAD 27), USDA NRCS soil series and percent slope as measured with a clinometer. Location refers to nearest landmark relative to UTM coordinates.Site Name Approximate Location Soil Slope Northing Easting
Series (% )____________________
Airport Flat Immediately North o f airport Cheme 4 to 6 4021688 727625
Callville Near old Callville Wash Road, on south Gypwash 1 to 3 4005353 700042Wash side o f A R 101
Cottonwood Just east o f Cottonwood Cove NFS Cove housing site
Huevi 2 to 3 3929876 708415
21
B
f . s!
L
' %- - -
-5 j- -■> - " c
Figure 7. Field photos of Callville Wash (A), Cottonwood Cove (B) and Airport Flat (C), respectively.
22
Field Methodology
This research is part of a broader study being conducted by Jeff Herrick (ARS
Jornada, Las Cruces, New Mexico) on the effects of ATV off-road use on dry and wet
soils. Wet soil sites were subjected to rainfall simulations for 30 minutes each at an
average of 107 mm hr using sprinklers underneath a tarp (Figure 8A). Rainfall
simulations were conducted from winter 2004-2005. Soil was sampled in May 2005
from three site locations in the Mojave Desert near the Las Vegas area: Cottonwood Cove
near Searchlight, Nevada; Callville Wash near Lake Mead Marina; and Airport Flat near
Lake Mead’s Dry gyp. The mean annual precipitation for all three of these locations is
between 10 and 15 centimeters and the mean annual temperature is between 22 and 23 °C
(NRCS Soil Series Descriptions). The digging of soil pits was prohibited due to
permitting regulations by the National Park Service and therefore no soil profile
descriptions were made below the uppermost (vesicular) horizon; however, series had
been previously identified at the study sites with the NRCS and Jornada ARS.
Soil samples were collected from undisturbed soils and from areas in the ARS
study where disturbed tracks were created by a quad all-terrain vehicle being driven once
over a plot. Off-road vehicle use is generally prohibited in the Lake Mead National Park,
and was permitted only for this study. In the ARS study, soils were excavated following
the rainfall simulation experiment to approximately 30 cm to observe wetting fronts
(Figure 8B). Holes were refilled with the same material removed. It is from these holes
that vesicular horizons were observed to reform and that were then sampled for this
current study; these soils with the reformed vesicular horizon are labeled disturbed.
23
A total of 15 (6 disturbed, 9 undisturbed) samples were taken from three holes in
eaeh of three plots at Cottonwood Cove and Callville Wash, ineluding the vesieular
horizon and an overlying silt eap (if present) for eaeh of the three holes. Only
undisturbed holes eontained a silt eap. Nine separate samples (6 undisturbed, 3
disturbed) from eaeh site were eolleeted as loose granular soil and sealed in plastie bags;
six samples (3 disturbed, 3 undisturbed) from eaeh site were eolleeted as intaet peds and
wrapped in foil with tops oriented upward for pore morphology analysis with a scanning
electron microscope. Airport Flat did not have a silt cap and therefore only 12 samples (6
disturbed, 6 undisturbed) were taken at this loeation. Subsequently, six (3 disturbed, 3
undisturbed) of these samples were eolleeted in plastic bags for physical and chemical
analysis and six (3 disturbed, 3 undisturbed) intaet, oriented ped samples were collected
using the methods deseribed above.
Laboratory Methodology
Pore Morphologv
Intaet, oriented vesieular ped samples to be analyzed for pore morphology via a
Scanning Electron Microscope (SEM) were sub-sampled and epoxied in Buehler Sampl-
Kup® reusable plastie eold mounting eups using an epoxy mix of 100 parts resin and 39
parts hardener and then eut into billets with a Hillquist SF-8 Trim Saw. Three peds were
made into billets and examined for eaeh disturbed and undisturbed set of samples. The
billets were sanded with silicon carbide grinding paper on an Eeomet variable speed
grinder-polisher to remove scratehes made by the saw. Samples were gold eoated in
order to allow for earbon analysis via SEM Energy Dispersive Speetroseopy (EDS). Pore
24
morphology was then examined on a Scanning Electron Microscope and with photo
imaging from SEM images (Lebron et ah, 1999; Krinsley et al., 1998). For each billet,
three pores were randomly chosen to be photographed and measured. Pores were chosen
for variety of shape and size in order to have a representative sample of all pores
observed. These images were imported into ImageJ image analysis software in order to
quantify the porosity within the sample. With this software, I measured the area of the
pores, pore perimeter, pore length and width, and pore roundness using the length to
width ratio. In order to explore pore orientation, I measured length along the longest axis
according to the orientation of the pore and measured the width along the shorter axis
perpendicular to the length (Figure 9), and then used these measurements to calculate the
length to width ratio, which determines the roundness of the pore and whether it is a
vesicle (rounded, smooth edged pore) or a vugh (irregular, rough edged pore). Pores
could also be classified as interstitial, or between grains; however, these pores were
removed from comparison by L-W'* ratio. I chose a length to width ratio value between
1.0 and 1.5 to indicate a vesicle while all other values would indicate a vugh. This range
of values was chosen based on the distribution of L-W* ratios (Figure 10), and the fact
that ratios closer to 1 would visually represent pores that are more round. Because L-W*
ratios may not always be an accurate portrayal of actual pore roundness, I also
determined whether a pore was a vesicle, vugh or interstitial by making judgments based
on my observations of the pore images (Figure 11) {following Brewer (1964)}.
Phvsical and Chemical Analvsis
Samples collected loose in the field were used to examine laboratory physical and
chemical properties hypothesized to lead to vesicle development. Samples were sieved
25
:
Figure 8. A. Rainfall simulator used preceding the study; B. Depth of wetting from rainfall simulations. Infiltration stops at the bottom of the vesicular horizon, which continues from the surface to the base of the hole pictured (~4 cm).
26
A
Figure 9. Examples of L W'^ ratio measurements based on pore orientation.
27
80 H Mean 1.869 StDev 1.047 N 18870-
60-
50-
40-
30-
20 -
10 -
0.0 3.0 4.5L W - 1
6.0 7.5 9.0
Figure 10. Histogram showing distribution of length to width ratios of all pores measured. The curved line shown is the normal curve (mean is at the peak of the curve).
28
Figure 11. Examples of a vesiele (A); a vugh (B); and an interstitial pore (C), all from Cottonwood Cove in the disturbed condition.
29
using 2 mm sieves (USDA Sand Classification) (Gee and Bander, 1986). To determine
the reactivity of the soil, pH was determined in 0.01 M CaCb and water (Soil Survey
Laboratory Staff, 2004). Conductivity (EC), used to measure salt content, was
determined with an Accumet basic conductivity meter (Soil Survey Laboratory Staff,
2004). Calcium carbonate was quantified by the manometer method (Carbonate
Equivalent (CCE): Williams, 1948; Horvath et al., 2005) using 6 M HCl reacted with a
soil sample. The pressure released by this reaction, measured in mmHg was then fit to a
regression curve which allowed me to quantify the percent of calcium carbonate in the
sample. Sand fraction separation was done and particle size distribution was determined
by the pipette method at Virginia Tech (Gee and Bander, 1986). Chlorophyll data,
collected by Herrick following the rainfall simulation experiments, was quantified in the
lab using High Performance Liquid Chromatography (HPLC) (Garcia-Pichel and
Castenholz, 1991) to measure the biological population in the cryptobiotic crust that is
commonly associated with desert surfaces and found in conjunction with the site
locations examined for this study.
All data were analyzed using either the Mann-Whitney (2-sample) or Kruskal-
Wallis (> 2-sample) nonparametric test; Minitab 14 (Minitab 2006) statistical software
was used for analysis. In statistics, two hypotheses, a null hypothesis and an alternative
hypothesis, are tested using a dataset. For this study, the null hypothesis, that there is no
significant difference between undisturbed and disturbed samples or between sites, was
tested against the alternative hypothesis that there are significant differences present
between samples. The aforementioned statistical tests generate a p-value, which
quantifies the strength of the evidence for or against the null hypothesis. The p-value is
30
often reported in terms of an alpha test, in which a value is selected for a and then is
compared against the p-value. If the p-value is equal to or smaller than a, the null
hypothesis is rejected; if the p-value is larger than a, the null hypothesis fails to be
rejected. For this study, an alpha of 0.05 was used to test for significance, but p-values of
<0.10 were also noted as potentially significant relationships for future investigation.
31
CHAPTER 3
RESULTS
Sample Site Descriptions
Field descriptions of the three sampling sites are presented in Table 2. The
sample site at Callville Wash contained a pavement with very tight (nearly 100% cover)
mixed lithology, consisting of small gravels. In the undisturbed sites, the vesicular
horizon was 3 cm (undisturbed 2), 3.5 cm (next to plot 6) and 5 cm (undisturbed 3) thick,
and was overlain by a 2 mm silt cap. The vesicular horizon in the disturbed sites was
sporadic and shallower than in the undisturbed sites (plot 6: < 2.5 cm, plot 7: 2-12 mm,
plot 8: 1-3 mm).
Cottonwood Cove exhibited a pavement with 100% clast cover of mixed sizes
consisting of mixed lithology. Vesicular horizons in the three undisturbed sites at
Cottonwood Cove (hole 1, 2 and 3) ranged from 4 mm to 4 cm in thickness and were all
overlain by a silt cap (1 cm, 2 mm, and 1-2 mm, respectively).
Pavement at Airport Flat covered approximately 90% of the surface and consisted
of petrocalcic fragments (samples violently effervesced with 0.1 m HCl). Vesicular
horizons at the undisturbed sites were 4 cm, 0.75 cm, and 3.5 cm thick, respectively.
Undisturbed holes 1 and 2 were associated with a biological crust. In disturbed plots,
vesicular horizons were 1-1.5 cm, 3.5 cm, and 2-4 cm thick, respectively. Based on field
observations alone, vesicular horizons at Airport Flat generally had smaller pores than
32
Cottonwood Cove and Callville Wash in both disturbed and undisturbed plots, except for
in the undisturbed hole 3 at Airport Flat, where the pores were more comparable in size
to the other two sites. Chlorophyll data indicate Airport Flat soils have the highest
chlorophyll content of all three sites (Herrick et al., 2005, personal communication).
Analyses Across Sites
Chemistry
Soil chemical analyses compared across sites using a Kruskal-Wallis multiple
comparisons test indicates no significant difference in pH across disturbed and
undisturbed sites. A Kruskal-Wallis multiple comparisons test indicates significant
(alpha = 0.05) differences in EC measurements across disturbed and undisturbed sites
between Cottonwood Cove and Callville Wash (Tables 3a and 3b), with Cottonwood
Cove samples always having a higher conductivity (mean = 2.9 pmhos cm"', and 0.73
pmhos cm"', respectively). There is a significant difference in %CCE between Callville
Wash and the other two sites across disturbed sites and between Cottonwood Cove and
Callville Wash across undisturbed sites; Callville Wash has the highest %CCE (mean =
18.5%, and 21.5%, respectively) of all three sites.
Particle Size Fractions
Means, standard deviations, and results of the Kruskal-Wallis multiple comparisons test
for particle size fraction data across sites, including percent sand, silt and clay, silt
fractions (coarse, medium and fine) and sand sieve fractions (very coarse, coarse,
medium, fine and very fine) are presented in Tables 4a and 4b. Results of the Kruskal-
Wallis multiple comparisons test indicate that Cottonwood Cove has significantly (alpha
33
Table 2. Description o f field sites.
Site Location Condition Silt cap depth Vesicular horizon depth
Airport Flat Hole 1 Undisturbed 4 cm < S.5 cm variableHole 2 Undisturbed S/4 cm <S.5 cm variableHoles Undisturbed 0.7 - 1.2 mm S.5 cm
H oles Disturbed none 1 - 1.5 cmHole 4 Disturbed none S.5 cmHole 6 Disturbed 2 mm 2-4 cm
Cottonwood Cove Hole 1 Undisturbed 1 cm S - 4 cmHole 2 Undisturbed 2 mm 4 cmH oles Undisturbed 1-2 mm 4 - 6 cm
Hole 1 Disturbed none 1 - 2 cmHole 2 Disturbed none 0 - 2 cmH oles Disturbed none 0 - 2cm
Callville Wash Hole 1 Undisturbed 2 mm S.5 cmHole 2 Undisturbed 2 mm S cmH oles Undisturbed 2 mm 5 cm
Hole 6 Disturbed none 2.5 cmHole 7 Disturbed none 2 -1 2 mmHole 8 Disturbed none 1-S mm
34
= 0.05) higher mean total silt (mean = 36.3 %, 51.4%) values and significantly lower
mean total sand values (mean = 41.3 %, 29.3%) than Airport Flat (mean = 15.4%, 19.8%
silt; 64.9%, 63.1% sand) for both disturbed and undisturbed sites (Tables 4a and 4b). A
significant difference exists for clay size between Airport Flat (mean = 17.2%) and
Callville Wash (mean = 22.0%) in the undisturbed sites; there were no significant
differences in mean clay values across disturbed sites.
Sand sieve analysis indicates that Callville Wash has significantly (alpha = 0.05)
higher mean very coarse (mean = 6.4%) and coarse (mean = 4.9%) fractions across
undisturbed sites, but not disturbed sites. Cottonwood Cove has significantly (alpha =
0.05) lower mean medium (mean = 3.7%) and very fine (mean = 13.9%) sand fractions
than Airport Flat in disturbed sites; no significant differences exist for these fractions
across disturbed sites. There are significant (alpha = 0.05) differences in the fine sand
fraction across both disturbed and undisturbed sites between Airport Flat (mean = 34.0%,
31.9%) and Cottonwood Cove (mean = 8.7%, 7.2%).
Within the silt size fraction. Cottonwood Cove has significantly (alpha = 0.05)
higher coarse silt (mean = 12.1%) in undisturbed samples and medium silt (mean =
21.1%) in disturbed samples than both Airport Flat and Callville Wash. Significant
differences also exist between Airport Flat and Cottonwood Cove for the coarse silt
fraction across disturbed sites (means = 0.17% and 9.25% respectively) and the medium
silt fraction across undisturbed sites (means = 12.6% and 27%, respectively). A
significant (alpha = 0.05) difference in the fine silt fraction was found between Airport
Flat and Cottonwood Cove across undisturbed sites (means = 4.75%, and 12.3%,
respectively); no significant difference was found in the fine silt fraction between either
35
Table 3a. Comparisons o f pH, EC, and CCE mean values across disturbed sites.
Site n pH EC CCE
Unitless pmhos cm"' %Airport Flat 3 8.4 ̂(0.27)*a 0.3 (0.2)ab 13.2 (2.8)aCottonwood Cove 3 8.0 (0.3)a 2.9 (2.2)a 12.3 (1.6)aCallville Wash 3 8.4 (0.2)a 0.2 (0.03)b 18.5 (1.6)bt means} standard deviations* Values in a column having the same leter are not significantly different at the a = 0.05 levd according to a Kruskal-Wallis multiple comparisons test.
Table 3b. Comparisons of pH, EC, and CCE mean values across undisturbed sites.
Site n pH EC CCE
Unitless pmhos cm"' %Airport Flat 3 8.2 ̂(0.1)*a 0.2 (O.Ol)ab 13.7 (0.7)abCottonwood Cove 3 8.6 (0.6)a 0.7 (0.6)a 11.8(2.4)aCallville Wash 3 8.3 (0.2)a 0.2 (0.03)b 21.5 (0.6)bt means} standard deviations* Values in a column having the same leter are not significantly different at the a = 0.05 levd according to a Kruskal-Wallis multiple comparisons test.
36
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of these two sites and Callville Wash, and no significant differences were found between
any of the sites across disturbed samples.
Pore Morphologv
Results showing means, standard deviations and results of a Kruskal-Wallis
multiple comparisons test for pore measurements (area, perimeter, length, width, and
length/width ratio) across disturbed and undisturbed sites are presented in Tables 5a and
5b. In the disturbed sites, Callville Wash was found to have a significantly (alpha = 0.05)
lower pore area (mean = 35.5 mm^) and perimeter (mean = 2.1 mm) than both Airport
Flat (mean = 1028 mm^, 9.5 mm, respectively) and Cottonwood Cove (mean = 904 mm^,
8.2 mm, respectively). No significant differences exist between the disturbed sites for
any of the other pore size parameters. In the undisturbed sites, Callville Wash also has a
significantly (alpha = 0.05) lower pore area (mean = 2.2 mm^) than Airport Flat (mean =
586 mm^) and Cottonwood Cove (944 mm^). Additionally, Callville Wash has a
significantly (alpha = 0.05) higher pore length (mean = 2.9 mm) and width (mean =1.6
mm) than the other two sites. However, no significant differences exist between pore
length to width ratios.
The length to width ratio was used to determine whether a pore was a vesicle or a
vugh. A vesicle is a perfectly rounded, circular pore while a vugh is more irregular
(Brewer, 1964). Results of the L-W* ratio indicate that 47% of all disturbed pores
examined are vughs and 53% are vesicles, while 57% of all undisturbed pores examined
are vughs and 43% are vesicles. At individual sites. Airport Flat has 45% vughs and 55%
vesicles at disturbed sites and 43% vughs and 57% vesicles at undisturbed sites;
Cottonwood Cove has 43% vughs and 57% vesicles at disturbed sites and 54% vughs and
38
46% vesicles at undisturbed sites; and Callville Wash has 56% vughs and 44% vesicles at
disturbed sites and 67% vughs and 33% vesicles at undisturbed sites (Table 6). Pores that
were classified as interstitial, or “between grains”, based on visual observation of SEM
images were removed from these results. Because 1 suspected that length to width ratio
measurements do not necessarily indicate true pore morphology (a pore may have a ratio
close to one but not be circular), 1 also analyzed visually whether a pore was a vesicle,
vugh, or interstitial based on visual observations of pore images, using pore images from
Brewer (1964) as guide. Analysis of pore images indicated that in undisturbed samples.
Airport Flat has 50% interstitial pores while both Cottonwood Cove and Callville Wash
undisturbed samples do not contain any interstitial pores. In disturbed samples,
interstitial pores at Airport Flat increase to 67%. Interstitial pores at Cottonwood Cove
increase to 44%, with a sharp decrease in vesicles from 22% in undisturbed samples to
9% in disturbed samples. Percentage of decrease in vughs was also greatest at Callville
Wash, from 78% in undisturbed to 47% in disturbed samples. Cottonwood Cove has the
smallest increase in interstitial pores, to 10%, in disturbed samples, with only an 8%
decrease in vesicular pores from 31% in undisturbed to 23% in disturbed samples (Table
7). A comparison between percent vesicles calculated from the length to width ratio
versus that calculated from observations of pore images indicate a 24% total difference at
Airport Flat, a 22% total difference at Cottonwood Cove, an 18% total difference at
Callville Wash and a 21% total difference for all sites. These results and results for
disturbed and undisturbed sites are presented in Table 8. In all cases, the percent of
vesicles calculated from the length to width ratio was always larger than the percent
calculated by pore observations alone.
39
Table 5a. Comparisons o f pore measurements across disturbed sites.
Site n Area
mm̂
Perimeter
mm
Length
mm
Width
mm
LW '
Unitless
Airport Flat 34 1028^1144)^a 9.5 (6.8)a 1 .8 (l.l)a 1.2 (0.9)a 1.7 (0.7)aCottonwood Cove 31 904(1017)a 8.2 (6.7)a 1.7 (0.9)a 1.2 (0.7)a 1.7 (0.6)aCallville Wash 32 35.5 (127.9)b 2.1 (l.O)b 2.3(1.2)a 1.4 (0.9)a 1.9(1.2)at meansÎ standard deviations* Values in a column having the same letter are not sigiificantly different at the a = 0.05 level according to a Kruskal-Wallis multiple comparisons test.
Table 5b. Comparisons of pore measurements across undisturbed sites.
Site n Area
mm̂
Perimeter
mm
Length
mm
Width
mm
LW '
UnitlessAirport Flat 29 586 ̂(594)*a 7.8 (5.7)a 1.8(1.3)a 0.9 (0.6)a 2.3 (1.8)aCottonwood Cove 35 944(1214)a 7.0 (6.2)a 2.1 (1.6)a 1.2(1.0)a 1.8(0.7)aCallville Wash 27 2.2(1.2)b 2.5 (l.l)a 2.9(1.4)b 1.6(1.0)b 1.9 (0.8)at meansX standard deviations* Values in a column having the same letter are not sigiificantly different at the a = 0.05 level according to a Kruskal-Wallis multiple comparisons test.
40
Table 6. Comparisons of vesicles versus vughs based on L W* ratios by pore orientation. Pores classified as interstitial by visual observation have been removed.
Site n DisturbanceClass
Vughs%
Vesicles%
Airport Flat 11 Disturbed 45 5514 Undisturbed 43 57
Cottonwood Cove 28 Disturbed 43 5735 Undisturbed 54 46
Callville Wash 18 Disturbed 56 4427 Undisturbed 67 33
All Sites 57 Disturbed 47 5376 Undisturbed 57 43
41
Table 7. Comparison of vesicles versus vughs versus interstitial pores based on visual observation of pore images, with reference to classifications by Brewer (1964). Vughs are irregular, closed pores; vesicles are circular, smooth edged pores; and interstitial are irregular pores in between grains.
Site n DisturbanceClass
Vughs%
Vesicles%
Interstitial%
Airport Flat 33 Disturbed 21 12 6730 Undisturbed 37 13 50
Cottonwood Cove 30 Disturbed 67 23 1036 Undisturbed 69 31 0
Callville Wash 32 Disturbed 47 9 4427 Undisturbed 78 22 0
All Sites 95 Disturbed 44 15 4193 Undisturbed 61 23 16
42
Table 8. Percent difference between pores classified as vesicles by the L-W* ratio versus pores classified as vesicles based on observation of SEM images. The L-W’* ratio always had a higher percentage of vesicles than were observed in the SEM images. For example, the difference of 19% in disturbed samples at Airport Flat is percent vesicles calculated by the L-W’* ratio minus the percent vesicles observed in SEM images.
Site DisturbanceClass
Difference%
Airport Flat Disturbed 19Undisturbed 28
Cottonwood Cove Disturbed 28Undisturbed 17
Callville Wash Disturbed 27Undisturbed 11
All Sites Disturbed 27Undisturbed 17
43
Analyses Between Disturbed and Undisturbed Soils
Chemistry
Soil chemistry data, including means, standard deviations and results of Mann-
Whitney tests between disturbed and undisturbed samples for individual sites and all sites
are presented in Table 9. Mann-Whitney results indicate no significant difference in pH
and EC between disturbed and undisturbed samples within individual sites. Within
Callville Wash, mean undisturbed CCE (mean = 21.5%) was found to be significantly
(alpha = 0.1) higher than in disturbed soil (mean = 18.5%).
Particle Size Fractions
Means, standard deviations and Mann-Whitney test results for soil particle size
fraction data (percent sand, silt and clay), silt fractions (CSI, MSI and ESI) and sand sieve
fractions (VCS, CS, MS, FS and VFS) between disturbed and undisturbed samples for
individual and all sites are shown in Table 10. The only significant (alpha = 0.1)
difference in particle size at Airport Flat was in the coarse silt size fraction, in which
undisturbed samples (mean = 2.49%) had a higher mean value than disturbed samples
(mean = 0.17%). There were significant (alpha = 0.1) differences in the silt and sand size
fractions, as well as in the very coarse, coarse, medium and very fine sand size fractions,
and in the medium and fine silt size fractions between disturbed and undisturbed samples
at Cottonwood Cove. In these cases, the total silt and the respective silt fractions
(medium and fine) all had higher mean values (means = 51.4%, 27.0% and 12.3%,
respectively) in the undisturbed samples while the total sand and its respective fractions
(very coarse, coarse, medium) had higher mean values (means = 41.3%, 11.0%, 6.8%
and 6.4%, respectively) in the disturbed samples. Very fine sand was significantly (alpha
44
= 0.1) higher in undisturbed samples (mean = 13.9%). At Callville Wash, there were
signifieant (alpha = 0.1) differenees in the silt size fraetion, along with the medium and
fine silt size fraetions, for whieh mean values were always higher in the undisturbed
samples (means = 24.6%, 14.7% and 7.0%, respeetively). Additionally, for all sites,
mean fine silt was found to be significantly (alpha = 0.1) higher in undisturbed samples
(mean = 8.04%) than in disturbed samples (mean = 4.75%). No other significant
differences in particle size were found between disturbed and undisturbed samples.
Pore Morphologv
Means, standard deviations and Mann-Whitney test results of pore measurements
(area, perimeter, length, width and length/width ratio) are presented in Table 11. There
are no significant differences (alpha = 0.05) in any of the pore measurements between
disturbed and undisturbed sites at Airport Flat or Cottonwood Cove. Significant
differences (alpha = 0.1) do exist between disturbed and undisturbed length 1 (mean =
2.0 mm, 2.7 mm) and width 1 (mean =1.4 mm, 2.0 mm) measures at Callville Wash.
Across all sites, the 1/w ratio at undisturbed sites (mean = 2.0) is significantly (alpha =
0.1) higher than at disturbed sites (mean = 1.8). No other significant differences are
observed in pore morphology between disturbed and undisturbed samples.
45
Table 9. pH, EC and CCE data for each o f the three and all sites, with comparisons o fmean values between disturbed and undisturbed samples.
Site nDisturbance
Class pH
Unitless
EC
pmhos cm '
CCE
%
Airport Flat 3 Disturbed 8.4 ̂(0.3)* 0.3 (0.2) 13.2 (2.8)3 Undisturbed 8.2 (0.1) 0.2 (0.01) 13.7 (0.7)
p-value 0.190 0.658 1.000
Cottonwood Cove 3 Disturbed 8.0 (0.3) 2.9 (2.2) 12.3 (1.6)3 Undisturbed 8.6 (0.6) 0.7 (0.6) 11.8(2.4)
p-value 0.19 0.663 1.000
Callville Wash 3 Disturbed 8.4 (0.2) 0.2 (0.03) 18.5(1.6)3 Undisturbed & 3 # ^ ) 0.2 (0.03) 21.5 (0.6)
p-value 1.000 1.000 0.081
All Sites 9 Disturbed 8.3 (0.3) 1.1 (1.7) 14.7(3.4)9 Undisturbed 8.4 (0.4) 0.4 (0.4) 15.7(4.6)
p-value 0.860 1.000 0.596t meansJ standard deviations* Values are significantly different at the a = 0.05 level according to a Mann-Whitney test.
46
1e
I
Ip3
T3
B
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II
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u
;
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oB
oo'r- so CS CO <s CSci- d- d d CSCO IT)
SO o CS o o o o 00 o OCO Tf o so o iri d Tf 00 d
fS IT)O 'O O (S fS
n IT) P o r i o
00 n o r-̂ 00 ^
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Tf fSSd-sg^ Os CO (N (N O
ü-m(N 00
"S
I
0 R G G Gd d d d d
o 00 CS r>* 00 Tj- lOc< o d Tj- o Tj- 00 CS
CS CS d d d
CS CS Tfo dOs so CS
CS OS os so <n TtOS d CO CS d Tf irj d
CS G 00 G pCO
COO s 00 so Tj- so CO oso 00 d SO iri r-
d CS d d
irT G ’ Gd
GCS d G
d d oo CS so O CSr~; p d iri so d 00 0000 r- d CS d
O ) IT) Tt od d00 00 o IT)Tf r-» o Tf p r- CS
so CO d so 00 so d
O ) CS m Tfd d00 soCO o O s O ) so CS CO
so CS d SO Tf d IT) CO d
CS o Ttd d d d r~.o 00 so CSCS o Tj- Tj- so OS Tj-
CS d d SO d so CO d
O ) CS COd GCS 00 CO o Osp CO 00 o Tf OS o so 00
OS o CS CO so 00 CSTf CS d so d Tt d
IT) 00 os G Gd IT)Tj- 00 s q 00 so OS 00so o Os Tj- o COCO IT) d CS d CS CO d
Tf o CSd d d d d doTi 00 OS o Os Tj- oscs d CO 00 CS d Os r-»CS d CS d CS d
“Q “Q (u
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47
Table 11. Pore measurements for each o f the three and all sites, with comparisons o fmean values between disturbed and undisturbed samples.
Site nDisturbance
Class Area Perimeter Length Width L W '
mm̂ mm mm mm Unitless
Airport Flat 3429
DisturbedUndisturbed
p-value
1028* (1144)* 586 (594) 0.2171
9.5 (6.8) 7.8 (5.7) 0.3888
1.8(1.]) 1.8 (1.3) 0.7564
1.2 (0.9) 0.9 (0.6) 0.2070
1.7 (0.7) 2.3 (1.8) 0.2411
Cottonwood Cove 3135
DisturbedUndisturbed
p-value
904(1017)944(1214)0.8775
8.2 (6.7) 7.0 (6.2) 0.4257
1.7 (0.9) 2.1 (1.6) 0.7873
1.2 (0.7)1.2 (1.0)0.9283
1.7 (0.6)1.8 (0.6) 0.1614
Callville Wash 3227
DisturbedUndisturbed
p-value
35.5 (127.9) 2.2 (1.2) 0.4514
2.1 (1.0) 2.5 (1.1) 0.1225
2.3 (1.2) 2.9 (1.4) 0.1733
1.4 (0.9) 2.2 (1.1) 0.1685
1.9 (1.2)1.9 (0.8) 0.4514
All Sites 9791
DisturbedUndisturbed
p-value
661 (988) 550.6 (903.9) 0.7752
6.6 (6.4) 5.9 (5.5)0.8933
1.9(1.1) 2.2 (1.5) 0.4690
1.2 (0.8) 1.2 (0.9) 0.9551
1.8 (0.9) 2.0 (1.2) 0.0535
t meansÎ standard deviations* Values are significantly different at the a = 0.05 level according to a Mann-Whitney test.
48
CHAPTER 4
DISCUSSION
Interpretation
Analyses across the three sample sites Airport Flat, Cottonwood Cove and
Callville Wash indicate differences in pore morphology that may be related to surface age
as well as differences in soil physical and chemical properties between each site.
Significantly higher EC and silt content in Cottonwood Cove samples and significantly
higher CCE in Callville Wash samples may provide stability for the larger percentages of
vesicular pores observed in undisturbed samples, as opposed to the larger percentage of
interstitial pores observed in Airport Flat samples, which have a higher percent sand
content and a lower EC and CCE. Pore size results indicate that sites with older surface
ages Airport Flat and Cottonwood Cove have significantly larger pore area and perimeter
measurements than the younger Callville Wash. The magnitude of this difference
between sites decreases with disturbance, indicating that an older geomorphic surface
may provide a higher level of stability for pores in an undisturbed soil within the
vesicular horizon.
Analyses between disturbed and undisturbed soils did not indicate any significant
difference in pore size (area, perimeter, length and width) with disturbance. Differences
in pore shape were found to occur between disturbed and undisturbed samples. The
percentage of interstitial pores were found to increase with disturbance while the
49
percentage of vesicles and vughs were found to decrease. The ability of a soil to re-form
vesicular pores following a disturbance may be related to the original physical and
chemical properties of that particular soil.
Pore Morphology
In this section, I will first discuss pore morphology results across sites, followed
by results between disturbed and undisturbed samples at each individual site as well as
for all sites combined.
Across Sites
Surface Stability and Pore Shape
Surface stability is an essential component of soil development; Wells (1982)
defined stability as the “rate of modification of landscape component of a given age.”
Research has shown that landscape modification with age is dependent upon the
geomorphic position of a surface, in which the most stable surface component is the
uplands where down cutting has not occurred allowing for longer times for soil
development; stability decreases at lower geomorphic positions where water has more
recently cut through the landscape (Connors et al., 1987). Applying these concepts to the
three sites in this study indicates that Airport Flat’s soils occur on the oldest surfaces and
Callville Wash’s on the youngest. Airport Flat is the oldest and highest topographic
surface (Mid Pliocene in age with Mississippian limestone source rock (Beard et al.,
2007)) with soils formed in alluvium from mixed rocks over semi-consolidated gravelly
sediments (NRCS Cheme Series Description). Cottonwood Cove, whose location has not
been formally mapped by the U.S. Geological Survey and not assigned a geologic age, is
50
believed to be the middle of the three in age based on aerial photos and field observations
of its geomorphic position within the landscape and pedogenesis. Cottonwood Cove’s
soils are formed from mixed gravelly alluvium consisting of volcanic and limestone
source materials (NRCS Huevi Series Description). Callville Wash, the youngest site and
lowest topographic surface, is Mid Pleistocene in age and formed in alluvium derived
mainly from limestone (NRCS Gypwash Series Description) with source materials
including a combination of Miocene volcanics and sedimentary rocks (Beard et al.,
2007).
Increased geomorphic stability of a surface should also indicate higher levels of
soil development, which would therefore result in a more developed vesicular horizon
(Anderson et al., 2002). To assess geomorphic stability as a predictor for vesicular
horizon development, field data on vesicular horizon thickness and surface properties
(chlorophyll content and percent stone pavement coverage) were collected from
undisturbed and disturbed soils. Based on past research, field indicators of greater
surface stability include a thicker vesicular horizon (Anderson et al., 2002), a higher
percent clast cover (Springer, 1958; Evenari et ah, 1974); Quade, 2001; Anderson et al.,
2002), and a higher chlorophyll content, indicating the presence of cryptobiotic crust
(Johansen, 1993; Eldridge and Greene, 1994; Belnap, 2004). Based on the age of the
geomorphic surfaces at the three study sites, the site with the best expressed vesicular
horizon in an undisturbed soil should be Airport Flat, followed by Cottonwood Cove and
finally Callville Wash. Field indicator results from the current study do not present a
clear relationship in comparison to geomorphic stability results. Field data indicate that
Airport Flat (oldest soils) has the thickest disturbed vesicular horizons and the highest
51
chlorophyll content in both undisturbed and disturbed vesicular horizons, supporting the
idea that greater aged surfaces with more stability have thicker vesicular horizons and
greater chlorophyll contents. However, field data also indicate that Cottonwood Cove
(middle aged soils) has the thickest undisturbed vesicular horizons, and, along with
Callville Wash (youngest soils), has 100% clast cover, while Airport Flat has
approximately 90% clast cover. Haff and Werner (1996) encourage caution when using
desert pavement data to predict surface age; pavements have been noted to be dynamic
due to external influences on stone displacement such as raindrop impact or animal
activity (Haff and Werner, 1996). Vesicular horizon age may also be independent of
surface age; in the Mojave Desert, on surfaces Pleistocene to Holocene in age, vesicular
horizon properties are similar from one surface to another while properties of the
underlying soil horizon material are very different from each other, indicating a Holocene
formation age for vesicular horizons throughout the entire area (McFadden, 1982).
Thermoluminescence dating of soil in the Cima Volcanic Field, California has indicated
that much of the vesicular soil material formed on Pleistocene surfaces is often Holocene
in age, accumulated as a result of climate change from a glacial to interglacial period
(McFadden et al., 1998). Levels of stability of the vesicular horizon at the three field
sites can therefore not be clearly determined by field data alone and must also be
examined using laboratory microscopic and chemical methods.
Vesicular horizon stability and thus pore stability are not only attributed to
geomorphic age and the length of time that pores have had to form or that surfaces have
had to accumulate materials. Pore stability can develop rapidly with repeated wetting and
drying cycles (Miller, 1971; Evenari et al., 1974; Figueira and Stoops, 1983) and be tied
52
to local climatic changes; with repeated wetting and drying cycles, pores become more
rounded, indicating higher stability within the horizon (Miller, 1971). The quick
formation period (within 3-4 months) for vesicles in this study following the rainfall
simulator experiment led me to hypothesize that sites with rounder pores prior to
disturbance should have rounder pores following disturbance. In this study, pore
roundness was determined by examining the length to width ratio of pores with a
scanning electron microscope. The length (longest axis of a pore) and width (shortest
axis) of a pore were measured and the ratio was calculated between length and width
measurements (L-W’’) to be used as an indicator of pore roundness (1 = ideal vesicle
shape). Pore roundness measures can also indicate whether a pore is a vesicle or a vugh;
vesicles have smooth curves and visually look like a circular pore while vughs are
irregular shaped pores (Brewer, 1964). Examples of vesicles versus vughs are shown in
Figures 12 and 13. Statistical results of pore length, width and the L-W'* ratio do not
indicate any relationship between disturbed and undisturbed pore roundness (Table 11).
To quantitatively determine whether a pore was a vesicle or a vugh, I established that a
vesicle would have a length to width ratio in the range of 1.0 to 1.5; all other values
would indicate a vugh. Because a classification of a vesicle or a vugh indicates a pore is
disconnected from all other pores, a pore may also be classified as interstitial, or
“between grains”, as is shown in Figure 14. I therefore removed all pores I determined to
be interstitial by observation of pore images (described below) before determining
percentage of vesicle to vugh based on the length to width ratio. Based on these rules, I
found that, not including interstitial pores. Airport Flat and Cottonwood Cove both have a
higher percentage of vesicles to vughs than Callville Wash in both disturbed and
53
undisturbed samples. Classification of a vesicle versus a vugh by length to width ratio
alone may result in misidentification of vesicles due to ratios being influenced by
connective channels included in the pore measurement. In Figure 12, two vesicular pores
are shown, but only the pore on the far left, with a ratio of 1.1, is classified as a vesicle
based on length to width ratio, while the central pore, with a ratio of 1.7, is classified as a
vugh because it includes an attachment that increases its overall length even though its
basic shape is that of a vesicle. Conversely, a vugh may have a similar length and width,
giving it a length to width ratio closer to one, even though it is not perfectly rounded, as
is shown in Fig. 15. Because of this, I also classified whether a pore was a vesicle, vugh,
or interstitial by visually observing pore images and making qualitative determinations
based on observed pore shape using pore shape descriptions by Brewer (1964) as a guide
(Table 7).
Results of visual pore observations from SEM images indicate that Airport Flat
contains the least vesicles and vughs and the most interstitial pores of all three sites in
both disturbed and undisturbed samples. Neither Cottonwood Cove nor Callville Wash
contain any interstitial pores in undisturbed samples, and Cottonwood Cove has the
fewest interstitial pores of all sites. Cottonwood Cove also contains the highest percent
of vesicles of all three sites, in both disturbed and undisturbed samples. Callville Wash
has the greatest increase in interstitial pores, and the greatest decrease in both vesicles
and vughs from the undisturbed to the disturbed condition. The high percentage of
vesicles and lack of interstitial pores found at Cottonwood Cove may be related to the
highly silty texture of Cottonwood Cove samples, which has been found in past research
to promote vesicular structure (Evenari et al., 1974; McFadden et al., 1998; Lebron et al..
54
2002), as well as high salt content (EC), which promotes dispersion of the soil (Musick,
1975) and thus could aid in vesicle formation. The high percentage of vesicles in
undisturbed Callville Wash samples may be related to carbonate cementation, which
strengthens and stabilizes vesicular structure (Evenari et al., 1974) (Callville Wash has
the highest CCE of all three sites) while the lower percentage of vesicles and higher
percentage of interstitial pores in disturbed samples may be related to a sandier particle
size not yet re-cemented by carbonate. Airport Flat’s high sand content paired with low
EC and CCE would therefore result in a higher percentage of interstitial pores in both
undisturbed and disturbed samples.
Pore Morphology
Disturbed vs. Undisturbed Soils
Hamerlynck et al. (2000) suggests that soil horizons that limit infiltration, such as the
vesicular horizon, will affect plant-water relations of the vegetation growing in these
soils. Therefore, how well the vesicular horizon reforms is of importance to land
managers conducting restoration ecology mitigation following land disturbance.
Restoration should achieve a soil functional result similar to conditions prior to
disturbance. Therefore, pores in this study should be morphologically similar before and
after disturbance. An objective of my research was to determine whether pore
morphology in the vesicular horizon is different between disturbed and undisturbed soils;
a difference in pore morphology could affect infiltration and thus affect what species
recolonize a disturbed area or if recolonization would even be possible. To examine the
55
Figure 12. Callville Wash UD pores with 1/w ratios of 1.1 and 1.7,1 to r (vesicle and vugh).
# rW
f
m e - f
miliAi
Figure 13. Callville Wash UD pore with 1/w ratio of 1.7 (vugh).
56
A" • -
Figure 14. Airport Flat D pore, classified as interstitial.
«
' .
;
»v :' V ' i
'V*-.V
.a';
1 ,
Figure 15. Airport Flat UD pore with L-W* ratio of 1.2 (classified as a vesicle)
57
effect of disturbance on pores, measurements were made of pore morphology (area,
perimeter, length and width) in undisturbed and disturbed soils. No significant difference
in pore morphology (area, perimeter, length and width) was found in this study between
disturbed and undisturbed soils. While there are not significant differences in pore size
with disturbance, results indicate that pore shape (vesicle vs. vugh vs. interstitial) is
different between disturbed and undisturbed samples. Disturbed sites have more
interstitial as opposed to closed vughs or vesicular pores, and would therefore be likely to
have increased water movement through the vesicular horizon to lower soil horizons.
Results discussed in the following sections are that of comparisons between undisturbed
and disturbed plots at individual sites as well as data composited across all sites (Table
10).
Pore Size and Shape
Previous researchers investigating vesicular pore morphology in the laboratory
versus samples of the horizon formed in the field have presented two different
explanations for pore size/shape changes over time: 1) in the laboratory, initial vesicle
formation and coalescence with repeated wet and dry cycles increases pore size (Figueira
and Stoops, 1983); and 2) field sample observation suggests repeated wet and dry cycles
lead over time to silt and clay accumulations around inner pore edges, which decreases
pore size (Sullivan and Koppi, 1991). Research therefore suggests that pore formation
differs over time as vesicle development occurs. Figueira and Stoops (1983) re-created
vesicles in the lab over 30 wet/dry cycles for an inferred time of 9-12 days (based on
three day drying times between each of 10, 20 and 30 wetting cycles - time was not
58
clearly stated). Sullivan and Koppi (1991) examined pores from already-established
vesicular horizons, although ages were not clearly specified.
Based on conclusions from Figueira and Stoops (1983) and Sullivan and Koppi
(1991), I hypothesized that pore size will be greater in a disturbed sample versus an
undisturbed sample, and in undisturbed samples accumulations of silt and clay material
around pore edges will cause pore size to be smaller. If pores become larger after being
destroyed and re-formed, as observed by Figueira and Stoops (1983) and pore size is
smaller in undisturbed samples, as observed by Sullivan and Koppi (1991), my results
should indicate that pores are always larger in disturbed than in undisturbed soils. Pore
morphology (area, perimeter, length and width) statistics indicate that pore area and
perimeter measurements are almost always higher in disturbed than undisturbed samples
(an exception is found at Cottonwood Cove where area measures are smaller in disturbed
samples) (Figure 16). However, results from length and width measurements do not
exhibit the same pattern as area and perimeter measurements, suggesting an influence in
pore shape that has not been accounted for. Pore morphologies that are dominated by
pores with rougher, more convoluted edges (more typical of vughs) versus smoother
edged pores (more typically vesicle shaped pores) will influence the outcome of pore area
and perimeter measurements calculated by the Imaged software. Differences in pore
shape determine whether the pore is classified as a vesicle, vugh, or interstitial. Pores
with such parameters resemble Figure 17, which has a pore morphology giving it the
appearance of an interstitial pore. Figure 18, which has a pore morphology of a smooth
edged, rounded vesicle, and Figure 19, which has a pore morphology representative of a
vugh. Results from Figueira and Stoops (1983), in which pores re-formed from
59
f ~ €2000 I
I - 1I IAirport Flat Cotlonwuoû
8
P o r e A r e a
m
f : BDiStUllJM . DUCidisturtJSd
1 1Cullwli» WasM Alt Sites ^
am ple S ites 1______ 1
P o r e P e r i m e t e r
" L Ü ;'!■ a = C A i
i : : -
a # # I 1 BOist'jfDed1 0 __ : 1 ^ 1 OUiga'iirW
' i l l r i É:Airport Ftîrl ColtonwwMJ Crr* Ofrll'JiiNi Wiish AiiSitoa
Sam ote S ites
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Airport Fiat Coüonwod
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m ple S ites ( /
P o r e W id th
-1̂ p - l j . ‘ -
L j y C . J ' C- ^ u - c . %
5 ^ ■ T: : '. r : ■■ : : ■ j : . r: ■ ■ ■ DtshitW fi , : 1 1 □ Uodiiiiurtwd
l ' i l IAirport Flat Cottonwood Cova Callvtie Wash All Sites
Sam ple S ites
P o r e L /W R a t io
U - : 1 3
è ^ ■■ M -: 1 - - ;[g I : — : " r r " 1 ‘ f 5 2 i 1 i l J ______ , I . OlMKUtew:
1 i r i ^Airport Fl;« (krtlonwcKxJ Co\e Call«i!« Wusli All Sii«!S 1 j - , 1
S am ple S ite s |_____ '_j
Figure 16. Pore size data for disturbed and undisturbed sites based on a Mann-Whitney test. Charts show difference between disturbed and undisturbed samples for pore area (A), perimeter (B), length (C), width (D) and length/width ratio (E).
60
previously disturbed vesicular horizons became interconnected with repeated wet and dry
cycles, resulting in larger, less rounded pores, suggest an increase in irregularly shaped
pores following disturbance. In the present study, the effect of disturbance on pore shape
is variable from one site to another. However, the percent of interstitial pores always
increases in disturbed samples. At least 70% of disturbed pores at Airport Flat (70%) and
Callville Wash (78%) have rougher edges and are more convoluted than undisturbed
pores, for which only 56% and 33% have rough edged pores, respectively. Cottonwood
Cove typically has smoother edged disturbed pores than the other two sites, and a less
pronounced difference in pore morphology as compared to undisturbed pores (30% rough
edged pores in disturbed vs. 15% in undisturbed).
Pore morphology statistical results suggest that evidence for a difference in pore size
between undisturbed and disturbed samples is marginal to non-existent (Table 10),
indicating that disturbance does not significantly affect pore morphology in terms of pore
size (area, perimeter, length and width). In terms of pore shape, disturbance increases the
percent of interstitial pores in all sites. Subsequently, the percent of vesicles and vughs
both decrease from undisturbed to disturbed samples, but the percent decrease in vughs is
higher than the decrease in vesicles (exception: Cottonwood Cove, where percent
decrease in vesicles is higher) (Table 7). In results based on pore L-W* measurements,
when interstitial pores are removed, the percent of vesicles increases in disturbed samples
(Table 6). From the perspective of restoration ecology, these results indicate that water
capacity held within vesicular pores in the disturbed condition should be similar to that of
the undisturbed condition; however, water movement through the pores may potentially
be altered with disturbance.
61
Figure 17. Convoluted-edged or interstitial pores, Airport Flat Disturbed.
V V "
n
- >- f i
Figure 18. Smooth edged ovoid pores. Airport Flat Undisturbed.
6 2
Figure Y9. kregul^^^sftaped..
dosed potG,CM V viUe^as.«ndis.-
63
Physical and Chemical Analyses and Pore Morphology
Across Sites
Results in this study suggest that differences in pore morphology between sites
may be related to soil particle size and chemical characteristics of the different soils.
Previous studies indicate that more rounded, better developed vesicles form in soils with
higher silt and clay content (Evenari et ah, 1974; Lebron et ah, 2002) and more elongated
vesicles form in sandy soils (Virto et ah, 2005). Particle size fraction analysis (major
fractions and silt and sand sub-fractions) from this study indicates that Cottonwood Cove
has a significantly higher mean total percent silt and lower mean total percent sand of all
sites in both disturbed and undisturbed samples (Figures 20, 21a). Cottonwood Cove also
has the lowest medium, fine and very fine sand, and the highest silt fraction values, also
in both disturbed and undisturbed samples (Figures 20, 21b). Airport Flat has the highest
total percent sand and Callville Wash has the significantly highest mean very coarse and
coarse sand values of all the sites for both disturbed and undisturbed samples. Therefore,
vesicular horizons at Airport Flat and Callville Wash should be less likely to form large,
rounded, well developed pores as compared to Cottonwood Cove. Pore area results
indicate that Cottonwood Cove does have the largest pores in the undisturbed condition.
Comparisons of percent vesicles versus vughs based on observations of pore images
indicate that Cottonwood Cove has the highest percent vesicles (roundest pores) of any of
the sites.
Precipitates, such as salts and CaCOg, can increase soil cementation and thus
increase the stability of pores in the vesicular horizon (Evenari et al., 1974; Anderson et
al., 2002). In this study significant differences in conductivity and %CCE are found
64
between sites. Cottonwood Cove has the highest mean EC value, and therefore the
highest salt content, of all three sites while Callville Wash has the highest CCE,
indicating the highest carbonate content. Callville Wash’s high CCE measures may be
due to carbonate parent material, eolian dust, or weathering of carbonate rock fragments.
According to the soil series description for Callville Wash (NRCS Gypwash Series
description), and field observations, soils are formed from alluvium derived mainly from
carbonate material, which is likely to affect the overall CCE value of the soil. In relation
to pore morphology, both Cottonwood Cove and Callville Wash have more rounded
pores than Airport Flat, in general. Salt and carbonate could therefore be potential
cementing agents, allowing for larger, more stable pores at these sites. However, further
research is needed to confirm this.
Physical and Chemical Analyses
Disturbed vs. Undisturbed Samples
If pore stability increases with precipitation of materials such as carbonates and
salt (Evenari et ah, 1974; Anderson et ah, 2002), assuming an undisturbed soil has had
more time to develop carbonate morphology than a disturbed soil, I hypothesized that
vesicular horizon pore morphology in an undisturbed soil may be dependent on
cementation by such materials. Conversely, I hypothesized that pore morphology in a
disturbed soil is largely dependent upon particle size because the time necessary for
cementation to occur would not yet have taken place in a soil disturbed only 3 to 4
months prior to re-formation and sampling. Results of chemical analyses indicate no
significant differences between chemical properties (pH, EC and CCE) in disturbed vs.
65
=1: ‘b-s d •• T £i b» I 5 : H
(ft 20 I- 3? I
U ndisturbed - Total Silt
bR
CoHoiw/oocJ Co« Sample Sites
.j -
OaiNdle Wash
OT ao i
111̂^Aiipwl Ffal CoRüTWWW} C(!W!
Sample Sites
D isturbed - C o arse Silt
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Sample Sites
U ndisturbed - C o arse Silt
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n
UndistljrbpH . Mnrljnm Silt
. J
I
* Values in a chart having the sam e letter are not significantly different at the a = 0.05 level according to a Kruskal-Vf/alfis com parisons test.
Figure 20. Total silt and silt fractions based on a Kruskal-Wallis Comparisons test.Charts show comparisons between sites for disturbed (A, C, E, G) and undisturbed (B, D, F, H) total, coarse, medium and fine silt fractions.
66
D isturbed - Total S and
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Sample SHbs
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1 ■Cottonwood Cove
Sample Sites
n
0
"Values in a chart having the sam e letter are not significantly different at the a - 0.05 level according to a Kruskal-Wallis com parisons test.
Figure 21a. Total sand and sand fractions based on a Kruskal-Wallis Comparisons test. Charts show comparisons between sites for disturbed (A, C, E) and undisturbed (B, D, F) total, very coarse and coarse sand fractions.
67
£ ^à? 30 i
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ab
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*Values in a chart having the sam e letter are not significantly different at the a = 0.05 level according to a Kruskal-Wallis com parisons test.
Figue 21b. Continued sand fraction data based on a Kruskal-Wallis Comparisons test. Charts show comparisons between sites for disturbed (A, C, E) and undisturbed (B, D, F) medium, fine and very fine sand fractions.
68
undisturbed samples within sites with the exception of carbonate content at Callville
Wash, in which the undisturbed mean CCE was significantly (alpha = 0.1) higher than the
disturbed mean CCE.
Particle size analysis indicates significant differences between undisturbed and
disturbed samples across all sites. While not always significant, all sites show higher
mean total silt and silt fraction values in the undisturbed samples than in the disturbed
samples. In contrast, higher mean total sand values are found in disturbed samples as
opposed to undisturbed samples; this is also true for all sand sieve fractions at
Cottonwood Cove, and most at Callville Wash, though only in fine sand and very fine
sand at Airport Flat. A difference in particle size in disturbed sites could be due to: 1)
natural variability within the site (having nothing to do with the vesicular horizon); 2) soil
being mixed in from a lower horizon during field analysis and sampling; 3) an actual
difference between the original soil texture and the type of material that reformed the
disturbed soil.
My hypothesis that larger, more rounded vesicles form in soil with higher silt and
clay contents should dictate larger and rounder pores in the undisturbed samples because
the soil prior to disturbance is finer grained. Yet based on this study’s results, no pattern
of change in morphology appears between disturbed and undisturbed samples in relation
to a change in physical or chemical properties with disturbance. This suggests that a
change in chemistry or particle size does not appear to affect pore re-formation, and
therefore, within this study, disturbance does not appear to affect overall pore size as
compared to undisturbed samples. Further research should examine the relationship
between texture and chemistry in the vesicular horizon due to the large standard
69
deviations found in this study. In terms of pore shape, results from observation of pore
images indicate that conductivity, carbonate content and particle size may affect whether
a pore is vesicular, a vugh, or interstitial. The soil with the siltiest texture and highest
EC, Cottonwood Cove, also forms the most vesicles, in both the disturbed and
undisturbed condition, while the sandier soils. Airport Flat and Callville Wash are less
likely to form vesicles. In undisturbed soils, cementation of soil as a result of high
calcium carbonate content at Callville Wash may promote the formation of vesicular
porosity.
Implications for Restoration Ecology
Although vesicular horizons are generally fragile and transient in nature, they help
to provide surface stability to the surrounding ecosystem via its unique pore morphology.
The lack of interconnected pores influence the rate and direction of water movement
through the soil. Results of this study indicate that a disturbance may lead to a change in
pore morphology in the vesicular horizon. A change in pore morphology has the
potential to alter soil water movement and thus overall ecosystem stability. It is
important for restoration ecologists to consider the effects of disturbance on the vesicular
horizon when working in desert landscapes.
70
CHAPTER 5
CONCLUSIONS
Results from this study indicate that surface and sub-surface (~30 cm) disturbance
did not have a significant effect on vesicular horizon pore morphology re-formation. The
development of pores in disturbed soils of this study only 3 to 4 months following a
disturbance confirms that rapid re-formation of pores observed in the laboratory
(Springer, 1958; Figueira and Stoops, 1983) also occurs in the field; other pore structures
occur as well (vughs, interstitial) and in some cases are more common than vesicles, as is
indicated in this study (Table 7). Whether a pore is vesicular, a vugh or interstitial in
shape following a disturbance is related to the physical and chemical properties of the soil
prior to disturbance, indicating that these properties may influence the re-formation of
pores in a disturbed soil. Only pore shape and size were analyzed for this study;
determination of changes in pore number and connectivity following a disturbance would
require further research. Further research is also needed to determine whether pore
morphology in disturbed soils will change further over time and if the changes that have
occurred will influence the ecology of the disturbed areas.
71
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75
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91
Chemical Analyses
Sample Name pH EC pmhos cm *
%CCE
1 Cottonwood Cove Hole 2 Bottom UD 8.7 0.43 9.432 Callville UD 2 Bottom Av 8.1 0.13 21.143 Apt Flat Plot 6 Dist Bulk 8.3 0.16 13.594 Apt Flat Hole 3 Undisturbed 8.3 0.18 13.795 Cottonwood Cove Hole 1 Bottom UD 8.0 1.37 14.26 Callville Plot 6 Dist 7 month 8.3 0.16 19.227 Callville Undist 2 Top Silt 8.3 0.18 20.718 Callville Undist Plot 6 Bottom Av 8.5 0.18 22.139 Callville Wash Undist PI. 6 area top silt 8.3 0.19 20.9510 Apt Flat Hole 2 Undisturbed 8.2 0.18 14.2311 Callville Undist 3 Top 8.4 0.19 14.912 Callville Plot 8 Disturbed 8.7 0.18 16.7313 Callville Undist 3 Bottom Av 8.5 0.16 21.1314 Cottonwood Dist Track Plot 8 7.8 4.49 14.0715 Apt Flat Undist Hole 1 8.1 0.20 12.9516 Cottonwood Hole 3 Av Bottom UD 9.1 0.39 11.8317 Airport Flat Disturbed Plot 4 8.8 0.55 10.2018 Cottonwood Dist Track Plot 2 7.8 3.78 11.7419 Callville Plot 7 Disturbed 8.3 0.13 19.6620 Airport Flat Disturbed Plot 3 8.3 0.14 15.7621 Cottonwood Cove Hole 2 Top UD 8.8 0.31 4.0422 Cottonwood Hole 3 Silt Cap UD 8.9 0.35 6.2523 Cottonwood dist track plot 7 8.4 0.33 11.1124 Cottonwood Cove Hole 1 Top UD 8.2 1.25 10.64
92
APPENDIX II
BACKGROUND ON ARID LANDS SOIL PROCESSES
IN THE MOJAVE DESERT
The development of arid soils is dependent upon many different faetors. These
faetors inelude the presenee and movement of water through the soil (Bolling and
Walker, 2002; MeDonald, 2002; Monger and Bestelmeyer, 2006), vegetation (Fonteyn
and Mahall, I98I; Bergelson, 1990; Andraski, 1997; Kemp et al., 1997; Van Breemen
and Finzi, 1998; Wileox et al., 2004; Hartle et al., 2006), sunlight (Austin and Vivaneo,
2006), stone eover or desert pavement (Springer, 1958; Evenari et al., 1974; Quade,
2001; Anderson et al., 2002), and the level of soil development (MeAuliffe, 1991; Parker,
1995; MeDonald, 2002). Though some proeesses may be more influential than others,
they all work together to ereate the eonditions neeessary for the formation and
development of arid soils, some of whieh may lead to vesieular horizon development.
These proeesses may work over the short-term (Bergelson, 1990; Van Breemen and
Finzi, 1998) or the long-term (MeAuliffe, 1991) and over the miero or maero seale
(Buxbaum and Vanderbilt, 2007).
One soil horizon unique to arid environments is the vesieular horizon, whieh is a
surfaee horizon with many non-intereonneeted pores. Of all the faetors neeessary for
vesieular horizon development, water is the main driving foree of all other proeesses that
lead to vesiele formation. Of the five soil forming faetors, elimate is the most important
93
in vesicular horizon formation because climate determines water movement. Water
movement in the form of rainfall influences overland flow, which influences topography.
Topography determines the type of organisms (vegetation, biological crusts, etc.)
growing on a surface. These plants and other organisms stabilize the surface, allowing
time for a vesicular horizon to develop, and also influence water uptake through
évapotranspiration (Christopherson, 2002). However, plant roots are detrimental to
vesicle formation; therefore, vesicular horizons are not prominent in well vegetated soils
(Anderson et al., 2002). The three above factors are all influenced by and changed
through time (Wells et al., 1987). Because vesicular horizons are mainly derived from
windblown dust (McFadden et al., 1986; 1992; Anderson et al., 2002), parent material
may also change through time. Vesicular horizons are able to form from a variety of
parent materials, with varying degrees of vesicular development, as is indicated in this
study.
In an arid climate sueh as that of the Mojave Desert, vesicular horizon
development often begins as dust aceumulation derived from desiccated pluvial lakes
(MeFadden et al., 1986; 1992). Gas displaeed from the soil by rainwater is neeessary for
vesicle development (Evenari et al., 1974). Desert pavement formation above a vesicular
horizon may both inhibit water movement into the soil and facilitate vesicle development
by trapping the soil gas that is displaeed by the rainwater (Springer, 1958; Evenari et al.,
1974). Water movement through the soil leaches carbonate, clay and other material that
contributes to soil cementation and vesiele stabilization (Evenari et al., 1974).
94
Effects of Soil Water Movement
The water that the formation of the vesieular horizon is dependent upon enters the
soil via moisture from the air and moves from the soil into the atmosphere through the
process of évapotranspiration. Transpiration involves the movement of water from the
soil into plant roots and out through their leaves (Christopherson, 2002), while
evaporation involves soil water turning into vapor (Brady and Weil, 2004); together this
mechanism is called évapotranspiration (Christopherson, 2002). Evapotranspiration is
temperature and humidity dependent; decreased temperatures and increased moisture in
the air will reduce évapotranspiration while increased temperatures and decreased air
moisture will cause évapotranspiration to increase (Christopherson, 2002). Potential
évapotranspiration is the amount of evaporation and transpiration that could take place
under ideal temperature and moisture eonditions; it is approximated using average
temperatures and day length (Christopherson, 2002). If all water is evaporated from a
surface but the potential évapotranspiration is still unmet, there is a deficit; the deficit
subtracted from the potential évapotranspiration is the actual évapotranspiration
(Christopherson, 2002). Deficits lead to water shortages, or droughts. If actual
precipitation is higher than the potential évapotranspiration, then there is a surplus, whieh
leads to overland flow in the form of lakes or streams, or to increased levels of
groundwater (Christopherson, 2002).
Surface water movement is particularly important to the development and
maintenance of vesieular horizons. Most researchers agree that vesicle formation occurs
when the soil undergoes repeated wetting and drying cycles (Springer, 1958; Evenari et
al., 1974; Figueira and Stoops, 1983). A study presented by Springer (1958) emphasizes
95
the importance of moisture movement in vesicular development. When he added water
to sieved soil, he observed the soil particles to rearrange themselves to hold water on top
for several minutes and then continue to rearrange themselves beneath the surface as
water infiltrated downward. Air bubbles in the soil were pushed to the surface where
they escaped and created voids that were eventually filled in. Upon repetition of this
process, depressions and vesicles were left in the soil where air had escaped (Springer,
1958).
Soil water movement or presence in arid regions, where vesicular horizons are
formed, is dependent upon seasonal rainfall and temperature (McDonald, 2002). As
compared to humid regions, for much of the year in arid regions of the Southwestern
U.S., precipitation is low and temperatures are high; therefore, infiltration is also low, and
much of the soil water is lost to evaporation (McDonald, 2002). In addition to low
rainfall, infiltration into arid soils may also be inhibited by the presence of a vesicular
horizon. Vesicular pores are not interconnected (Figueira and Stoops, 1983; McFadden
et al., 1998), which reduces infiltration of water at the surface (Young et al., 2004), and
therefore to lower soil horizons. The amount of soil water evaporated is controlled by the
depth of infiltration into the soil, which is dependent upon amount of rainfall (McDonald,
2002). Seasons of increased rainfall and decreased temperatures result in deeper
infiltration depths, increasing overall transpiration (McDonald, 2002). The amount of
rainfall received in a given area versus another may also have significant effects on the
extent of development of vesicular soil pores, as is observed in this study.
Within an arid environment such as that of the Mojave Desert, the amount of
atmospheric moisture received by the soil from rainfall is very important in pedogenesis
96
as well as vesicular horizon development. Rainfall data for the study area was therefore
evaluated for the year during which the study took plaee to determine any patterns that
may indicate an influenee on the vesieular horizon development as observed in this study.
Preeipitation data for eaeh of the three sample sites for this study, Callville Bay (Callville
Wash), Cottonwood Cove and Echo Bay (Airport Flat), was provided by Mark
Sappington, GIS Speeialist at the Lake Mead National Reereational Area. Results of
these data indieate that in the one year span between the rainfall simulations in May 2004
and sample eolleetion in May 2005, there were some major differences recorded in
rainfall data between the three sample sites, particularly between Cottonwood Cove and
the other two sites. Airport Flat and Callville Wash. Rainfall at Callville Wash was
reeorded to have oeeurred on 35 days out of the year, in whieh the lowest level of
preeipitation, 0.02 inehes, oeeurred on August 19, 2004; November 23, 2004; Deeember
27, 2004; and Mareh 5, 2005, and the highest preeipitation of 2 inehes oeeurred on
Deeember 29, 2004. At Airport Flat, preeipitation was reeorded on 36 days out of the
year, in whieh the lowest rainfall of 0.02 inehes oeeurred on November 28, 2004, and the
highest rainfall of 1.93 inehes oeeurred on Deeember 29, 2004, whieh was also the date
of highest recorded rainfall for Callville Wash. As these two sites are loeated within
relatively elose proximity to eaeh other, it is likely that preeipitation at both sites eame
from the same rainfall event. Finally, at Cottonwood Cove, preeipitation was only
recorded on 9 days out of the entire year, in which the lowest precipitation during that
period, reeorded at 0.02 inehes, oeeurred on January 26, 2005, and the highest
precipitation, at 0.5 inches, occurred on August 15, 2004. In addition to preeipitation,
relative humidity is also an important indieator of the amount of moisture in the air.
97
Relative humidity in Las Vegas varies from month to month; however, the average
annual relative humidity through 2006 was recorded to be at 39% in the morning and
21% in the afternoon (The National Climatic Data Center, NO A A). However, these data
only apply to local and current climate conditions. Climate throughout the present
Mojave Desert has changed significantly since the Pleistocene, and continues to be
variable throughout the region.
Effects of Climate Change
Precipitation levels differ with climate change through time and space and thus
soil development and the presence or absence of horizons like the vesicular horizon will
differ with changing climate; past climatic influences are often preserved in arid soils. In
the present climate, the Mojave Desert region typically consists of long, dry periods with
short, scattered rainstorm events (Bolling and Walker, 2002). Research indicates that the
previous Mojave Desert climate was much different than it is today (VanDevender and
Spaulding, 1979; Peterson, 1980; Spaulding, 1982; Spaulding et al., 1983; Schlesinger,
1985), which also caused differences in soil properties, such as wetting depths and
carbonate and clay translocation (McFadden and Tinsley, 1985). Higher levels of
precipitation during the Wisconsin glaciation caused carbonates to be leached to lower
than 100 cm, greater depths than they are able under drier conditions (Schlesinger, 1985).
Additionally, plant macrofossil data, including evergreen oaks, yuccas, and some
warmer-desert elements such as barrel cactus, indicate that, south of latitude 36°N, desert
climate in the American Southwest during the latest Pleistocene had milder, moister
winters and cooler, drier summers than there are today (VanDevender and Spaulding,
98
1979; Spaulding, 1982; Spaulding et al., 1983). Argillic horizons were also much more
prevalent in the wetter Pleistoeene than they are in the present environment (Peterson,
1980). Researeh indieates, however, that ideal elimate eonditions for modem vesieular
horizon formation oeeurred during the Holoeene, in terms of desert pavement formation
(Quade, 2001) and soil aeeumulation rates (Reheis et al., 1995). In his study of the
Mojave Desert, Quade (2001) found well-developed vesicular horizons up to ~1560 m,
along with some vesieular development up to 2000 m. Given Quade’s (2001) hypothesis
that, at elevations between approximately 400 and 1900 m, desert pavements are only
formed during interglaeial periods, the vesieular horizons he observed at these elevations
must be Holoeene in age. Previous workers (e.g., McFadden et al., 1986; Anderson et al.,
2002) studying the origins of the vesieular horizon and its formation provide further
evidenee for this hypothesis.
Prior to the development of vesieular pores, the soil that will eventually eomprise
the vesicular horizon is accumulated as dust (Anderson et al., 2002), which, in the present
Mojave Desert, aecumulated in the Holoeene from material from dry lake beds or playas
(McFadden et al., 1986). Drill eore data results indieate the presence of Holoeene lake
deposits in the Silver Lake playa of the Mojave Desert, which was previously thought to
not have supported lakes sinee before 8000 yrs BP (Enzel et al., 1989). Radioearbon
dating indieated a eorrespondenee of these lakes to the early Neoglaeial (3620 ± 70 yrs
BP) and to the Tittle iee age’ (390 ± 90 yrs BP) (Enzel et al., 1989). Based on evidenee
indieating a link between the atmospherie conditions over the North Pacifie and the
Mojave River floods that produeed the lakes, Enzel et al. (1989) suggest that the
formation of the lakes oeeurred as a result of similar atmospheric circulation patterns to
99
the modern elimate of the North Paeifie. Soil development as a result of dust
aeeumulation is pulsatory, eoineiding with periods of aridity, playa development and
increased eolian aetivity (MeFadden et al., 1986; Chadwiek and Davis, 1990). Results
from a study by Reheis et al. (1995) indieate shifts from high to low soil aeeumulation
rates throughout the Pleistoeene and Holoeene, with a eonsistent inerease in aeeumulation
oecurring at the 50-15 ka Pleistoeene to Holoeene boundary, and that modem silt and
elay aeeumulation rates are 1.3 to 9 times greater than those of the late Holoeene. These
shifts in dust flux and soil aeeumulation are suggested to result from a threshold that is
reaehed when silt, elay and CaCOs are suffieiently aecumulated to decrease aeeumulation
rates and ereate conditions ideal for chemical weathering (Reheis et al., 1995). The
presenee of well developed soils (ineluding vesieular, or Av, and B horizons) in the
Mojave Desert provide additional evidenee for soil aeeumulation in the reeent Holoeene
following the desieeation of playa lakes and rapid deposition and entrapment of eolian
dust beneath a desert pavement (MeFadden et al., 1992).
In addition to the influenee of time on soil formation, elimate varies regionally
aeross southem Nevada. Inereased elevation in southem Nevada moving from the south
to the north marks the transition between the Mojave Desert to the south and the Great
Basin Desert to the north (Beatley, 1975), and beeause of this transition, the amount of
rainfall reeeived throughout southem Nevada is variable based on the loeation of the
basin. For example, Frenehman Flat, a elosed drainage basin in the Mojave Desert,
reeeives a mean preeipitation of approximately 130 mm, while Yueea Flat, a elosed
drainage basin in the marginal Great Basin (elevational transition between Mojave and
Great Basin Deserts), reeeives approximately 180 mm of rainfall, and so on moving
100
towards the north and into the Great Basin Desert (Beatley, 1975). Amount of rainfall is
very important in determining the type of soil and vegetation present in arid climates such
as that of the Mojave Desert.
Effects of Vegetation
Vegetation in soils containing vesicular horizons is generally very scarce due to
lack of water, and is mainly concentrated in depressions or rills, which contain runoff
(Evenari et al., 1974). Plants in arid climates are called xerophytes, from the Greek
words xeros (dry) and phytein (to grow) (Evenari, Shanan and Tadmor, 1982).
Xerophytes are drought-resistant, waxy, hard-leafed, and have low loss of water due to
transpiration (Christopherson, 2002). Transpiration accounts for much of the soil water
loss in arid systems (Kemp et al., 1997). Andraski (1997) suggests plants may be
inhibiting soil water depth and continued penetration and storage in two ways: 1) By
controlling the annual near-surface water balance via the natural soil-plant system. 2)
Plant activity in the uppermost soil layer acts as a second control on downward water
movement, resulting in episodic periods of extremely dry soil during times of below-
average precipitation. This influence is found to be dependent on changing temperature
gradients in the summer and the winter (Andraski, 1997). Differences in soil-plant water
relations have been found to occur on different types of surfaces during the dry summer
season, but not during the wetter winter-spring season (Smith et al., 1995). In his study
of three different surface types northwest of Las Vegas, including a Wash, an alluvial fan
remnant (Bench), and a mountain slope. Smith et al. (1995) observed that Bench
(shallow, fine textured soils) plants were completely dormant in mid-summer while Wash
101
(deep, coarse-textured soils) plants still exhibited some stomatal opening in the early
morning before high levels of evaporation/transpiration occurred. These observations by
Smith et al. (1995) are examples of miero seale vegetation differences across the
landscape.
Controls on vegetation can work on both the micro and macro scale. Macro scale
(regional) vegetation is mainly eontrolled by climate while micro scale (landscape)
ehanges in vegetation are controlled by variation in the soil moisture regime, soil depth,
developmental age and differenees in the loeal topography (Buxbaum and Vanderbilt,
2007). Other influences on mieroelimate in relation to the soil include slope aspeet,
elevation and lateral water redistribution (Monger and Bestelmeyer, 2006). These
influences on mieroelimate ean also affeet the soil itself, as is exemplified by the
sometimes patehy or random distribution of the surfaee vesieular horizon observed in the
field. Beeause all three sample sites in this study are relatively close to one another and
thus all located within an area having a similar climate, one would not expect large
differenees in the maeroclimate, or regional, scale between the three sites. Micro scale
differences between the three sites are more likely, and will therefore be diseussed
further.
An example of differenees in micro-soil climate is found in what are called fertile
islands. In arid environments, indivdual shrubs contain their own islands of soil fertility
(Thompson et al., 2005). Fertile islands are areas of transition in the soil
mieroenvironment that extend from beneath a shrub eanopy cover to bare shrub
interspaees (Bolling and Walker, 2002), eonsist of leaf litter (Thompson et al., 2005), and
help to promote seedling establishment and plant growth (Fowler, 1986; Hamriek and
102
Lee, 1987; Facelli and Pickett, 1991; Pugnaire et al., 1996; Van Breemen and Finzi,
1998). Fertile islands are formed as a result of biological causes (Gamer and Steinberger,
1989) and are affected by shrub processes, decomposition and animal transport of organic
materials (Bolling and Walker, 2002). The extent and development of the fertile island is
species dependent, but is generally related to the extent of the plant canopy cover
(Thompson et al., 2005).
Soil properties, such as the presence of a vesicular horizon, are heterogeneous
between plant canopy and intercanopy spaces within a landscape (Breshears et al., 1998).
Due to the effects of litter insulation and changes in the angle of the sun, a temperature
reversal was indicated in which intercanopy spaces were found to have higher soil
temperatures than canopy areas during warmer months but lower temperatures during
cooler months (Breshears et al., 1998). Differences in soil moisture between the two
microclimates have also been observed in which water input beneath canopies is reduced
relative to that of intercanopy spaces during cooler months due to increased uptake by
plants, while water loss is reduced beneath canopies relative to intercanopy spaces during
warmer months due to lower rates of evaporation and runoff (Breshears et al., 1998).
These differences in both soil temperature and evaporation rates can have effects on the
biological processes, including germination rates, species distribution and soil moisture
availability, of the different types of species growing in this environment (Breshears et
al., 1998). Due to the horizontal heterogeneity between canopy and intercanopy patches,
these areas are affected differently by soil temperature, moisture and evaporation as a
result of the presence of herbaceous and woody plants (Breshears et al., 1998), and can
have different properties that affect the soil in various ways.
103
Interspace surfaces between fertile islands sometimes consist of biological crusts
(Thompson et a l, 2005), which are soil crusts made up of cyanobacteria, lichens and
mosses, and are associated with many desert surfaces in which vesicular horizons are
present, and thus may affect or be affected by vesicular horizon development (Belnap et
al., 2004). Biological crust cover varies with changes in elevation and type of vegetation
(Thompson et al., 2005). While soil organic matter increases with increasing elevation,
biological crust cover is found to decrease (Thompson et al., 2005). Some species such
as white bursage allow for crust growth beneath their canopies as well as within
interspaces, while higher amounts of leaf litter have been found to inhibit biological crust
growth beneath creosote canopies (Thompson et al., 2005). Climate is also an important
factor in biological crust growth. Arid conditions are more stressful to microbial
communities than humid conditions (Li and Sarah, 2003). In arid environments,
microbes are only active when water is available (Li and Sarah, 2003). However, in the
upper soil horizons, microbial biomass enzyme activity was found to be higher in arid
than sub-humid regions (Li and Sarah, 2003). Li and Sarah (2003) hypothesize that the
efficiency of the metabolization of organic matter in arid environments during the short
time that water is available allows for a sufficient number of enzymes to complete soil
metabolism and nutrient cycling.
The influence of microbes may also be a factor in the development of vesicular
pores. Several different processes, including air expansion (Springer, 1958; Brewer,
1964; Evenari et al., 1974), wetting fronts (Springer, 1958) and the release of microbial
CO2 (Paletskaya et al., 1958), have been proposed as the leading cause of vesicle
development in the vesicular horizon. A common theme exists throughout the
104
hypotheses for vesicle development by Springer (1958), Paletskaya et al., (1958), Brewer
(1964) and Evenari et al. (1974) in that all authors agree that vesicles are formed as a
result of gas displacement in the soil. The question is whether the cause of this gas
displacement is abiotic or biotic in origin. Previous researchers (e.g., Geron, 2006;
Stevenson and Verburg, 2006) have examined some of the different sources of gas
movement in soil. Geron (2006) found climate and temperature conditions to have a
significant impact on soil gas formation. Soil surface temperatures have the potential to
exceed air temperatures by greater than 20°C, and certain plant species also have leaf
temperatures significantly above that of the air temperature, which can impact biogenic
non-methane volatile organic compound emissions (Geron, 2006). The amount of
rainfall that an area receives may also be of importance to the amount of gas displaced in
the soil. In addition to the displacement of trapped soil air by rainwater as proposed by
Evenari et al. (1974), increased rainfall also leads to increased litter biomass, which in
turn increases isoprene and monoterpene (biogenic volatile organic compounds)
emissions more so than the effects of higher temperatures (Geron et al., 2006). The
release of CO2 can have either organic or inorganic origins (Monger et al., 1991;
Stevenson and Verburg, 2006). The release of microbial CO2 has been found to
contribute to the formation of pedogenic calcite (Monger et al., 1991 ; Amott and Pautard,
1970), and has been suggested by Paletskaya et al. (1958) to cause the formation of
vesicles. Results from Stevenson and Verburg (2006), in which CO2 production from
carbonate dissolution is responsible for at least 13% of the total respiration in arid soils,
indicate the importance of abiotic carbon sources in arid systems. However, Springer
(1958), Evenari et al. (1974) and Brewer (1964) hypothesize vesicle development is
105
caused by either trapped air or the expansion of heated air in the soil beneath a stone or
crust seal. Evenari et al. (1974) disagree with Paletskaya et al. (1958), who suggest that
CO2 release from the drying of the bicarbonate in soil solution, along with precipitation
of carbonate from solution, is the main cause of vesicle formation. The hypothesis for
the presence of algae as a cause for vesicle formation (Paletskaya et al., 1958) is also
rejected by Evenari et al. (1974). They argue that characteristics such as the presence of
CaCOa and algal crusts contribute to the stability but are not the cause of vesicle
development.
Another important factor to consider in plant-soil relationships, and this formation
of vesicular horizons, is the influence of plant root growth. The relationship between root
growth and soil moisture is species dependent (Wilcox et al., 2004; Hartle et al., 2006).
Results of research by Wilcox et al. (2004) indicate that two desert plant species.
Ambrosia and Ephedra, had root growth which was positively correlated to soil moisture,
while one species, Larrea (creosote), had root growth which was negatively correlated to
soil moisture (Wilcox et al., 2004). Creosote root systems were also found to be deeper
and more extensive than the root systems of other arid land species, such as A. dumosa
(white bursage) and L. pallidum (pale desert-thorn), which maintain their underground
zone of influence closer to their stem base (Hartle et al., 2006). The different foraging
strategies observed between plant species may be related to the amount of rainfall
received in a given area; sites with Ambrosia and Ephedra received higher amounts of
precipitation while sites containing Larrea received lower amounts (Wilcox et al., 2004).
Old creosote bush root systems have been found to act as preferential flow paths that
allow for increased infiltration down through the soil profile (Devitt and Smith, 2002).
106
Larger plant size increases variability of water movement (Devitt and Smith, 2002). As a
result of the presence of desert shrubs, évapotranspiration is increased and drainage is
minimized (Devitt and Smith, 2002). Creosote bush root depth closely corresponds to
depth of penetrating soil moisture (Wallace and Romney, 1972). Creosote root growth is
found to be dependent upon the presence and depth of a petrocalcic horizon and the
intensity of rainfall that an area receives (Cunningham and Burk, 1973). A soil with a
shallow petrocalcic horizon will inhibit root growth to lower depths beyond the
petrocalcic. In these areas, a light rainfall is more beneficial to the creosote bushes,
whose roots are better able to receive moisture from shallower depths (Cunningham and
Burk, 1973). Shrubs in areas that do not have a petrocalcic horizon, or where it is deeper,
are able to extend their roots to lower depths and are thus better equipped to receive
moisture from heavier rainfall events, in which water percolates lower down in the soil
profile (Cunningham and Burk, 1973). The presence of a cryptobiotic crust may enhance
root growth and mineralization due to warmer soil temperatures beneath the crusts (Evans
and Johansen, 1999). Further study is necessary, but interactions between mycorrhizal
fimgi and soil crust forming organisms may also influence plant growth (Pendleton et al.,
2003). Evidence for the presence of roots in vesicular horizon samples is rare (Anderson
et al., 2002) due to the hypothesized potential disturbance, or inhibition of, vesicular
structure development by root growth. However, if a plant were to be removed from a
location with soil conditions otherwise conducive to vesicular horizon development,
vesicular pores would be able to form in its place.
The rapid destruction and re-development of vesicular pores, as observed in this
study, provides evidence for the versatility of this horizon through time. Soil-plant
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relationships are also influenced by time, over both long (MeAuliffe, 1991) and short
term (Bergelson, 1990; Van Breemen and Finzi, 1998) seales. Annual plant distribution
is expeeted to change from one generation to the next (Bergelson, 1990; Van Breemen
and Finzi, 1998). And long-term ehanges (over several millennia) have been observed in
the Sonoran Desert (MeAuliffe, 1991) and are expeeted to oeeur in the Mojave Desert as
well (Bolling and Walker, 2002). The spatial distribution of plants ean have a dramatie
influenee on competitor seedling produetion (Bergelson, 1990). Bergelson (1990) found
that when Poa (an annual grass speeies) was patehily (vs. randomly) distributed, invading
speeies Seneeio and Capsella seedlings inereased by over 400% in the next generation.
Thus the distribution of the vesieular horizon is in part affeeted by the distribution of
plants in arid environments.
Inter- and intra-species eompetition also influences the spatial distribution of
plants, whieh varies depending on speeies and loeation. In a study of the relationship
between vegetation spaeing and amount of preeipitation in the North Ameriean desert by
Woodell, Mooney and Hill (1969), the density of ereosote populations was higher in
areas of higher rainfall than in areas of lower rainfall. Additionally, the amount of
rainfall determined whether the horizontal pattern of ereosote was elumped (in areas of
high rainfall) or regularly spaeed (in areas of low rainfall) (Woodell, Mooney and Hill,
1969). Based on these findings and two theories from previous workers that diseuss the
relationship between plant cover, root growth and rainfall in arid regions (Walter, 1962;
Pielou, 1959), two widely aeeepted (Fonteyn and Mahall, 1981) eonelusions were made
about plant distribution in arid environments: 1) Plant eover deereases with subsequent
lateral root growth and 2) In areas of low rainfall, regular spaeing of the vegetation is due
108
to root competition for available water (Woodell, Mooney and Hill, 1969). Fonteyn and
Mahall (1981), however, suggest these conclusions are not definitive, as they are mainly
based on ecological theory when information based on carefully designed, controlled
field experiments is needed. Field experiments by Fonteyn and Mahall (1981) indicate
that interference between different species is more intense than interference within the
same species. Between-species interference may result from competition for water or
from interactions between roots (Fonteyn and Mahall, 1981). However, some
interference within the same species does exist to different extents. Of the two species in
their study, Fonteyn and Mahall (1981) attribute differences in distribution between the
species at least partially to intra-species interference. Larrea, which is more regularly
distributed, has more interference within its own species than Ambrosia, which is more
contagiously distributed (Fonteyn and Mahall, 1981). Longevity of species also must be
taken into account in that a longer lived species, such as Larrea, has more time to form a
regular distribution of plants than a shorter lived species, such as Ambrosia (Fonteyn and
Mahall, 1981). Results of these studies by Fonteyn and Mahall (1981) indicate that many
different factors influence the resulting properties of the soil and vegetation in an area
and must therefore be considered when studying the genesis of soils and horizons like the
vesicular horizon.
Other Factors
In addition to water, climate and vegetation, other factors can also influence soil
processes and properties. A particularly important factor in the Mojave Desert, especially
to the surface vesicular horizon, is the effect of sunlight. The direct effect of solar
109
radiation is found to be more important to litter decomposition than other controls, such
as water availability (Austin and Vivaneo, 2006). In a study where all variables exeept
for solar radiation were eliminated, there was a loss of 33% to 60% organie matter over
time (Austin and Vivaneo, 2006). Arid and semi-arid eeosystems may be more
suseeptible to the breakdown of above-ground organie material by solar radiation than in
humid elimates beeause of high radiation intereeption by the soil surfaee, large amounts
of standing dead plant material and low rainfall (Austin and Vivaneo, 2006). However,
the effeets of sunlight, as well as other faetors, may be somewhat inhibited in areas due to
desert pavement, a stone covering eommonly present in areas eontaining vesieular
horizons.
A desert pavement develops above vesieular horizons as soil material aeeumulates
as dust beneath a stone surfaee (Anderson et al., 2002). The desert pavement deereases
infiltration, which increases when a vegetative eover, also reduced by the presence of a
desert pavement, is present (Abrahams and Parsons, 1990). A higher pereentage of desert
pavement is an indicator of increased soil surface development (Springer, 1958; Evenari
et al., 1974; Quade, 2001 ; Anderson et al., 2002). On a hillslope in the semiarid region of
southem Arizona, infiltration rate is positively eorrelated with shrub eover and negatively
eorrelated with stone cover in the shrub interspaees (Abrahams and Parsons, 1990).
These results are influenced by the presence of shrub eover. When infiltration is
eorrelated with stone cover alone, there is a positive eorrelation; it is only when
infiltration rate is evaluated beneath shrubs as well as in the interspaces that there is this
opposite eorrelation between infiltration rate and type of cover present (Abrahams and
Parsons, 1990). Infiltration rate is greater beneath shrubs due to coarser grain size, higher
110
amounts of organic material, increased burrowing by animals and the greater inhibition of
surface sealing by plants versus surface gravel (Abrahams and Parsons, 1990). Gravel
cover has been found to slightly increase water retention, and thus transpiration rates,
relative to soils without a gravel cover (McDonald, 2002). Areas having a higher amount
of gravel cover have also been found to inhibit evaporation in favor of internal drainage
more so than areas with less gravel cover (Andraski, 1997).
A well-developed desert pavement, charaeterized by a high percent clast cover, is
a frequently used indicator of an advanced soil-landscape stability and perhaps
development (Springer, 1958; Evenari et al., 1974; Quade, 2001; Anderson et al., 2002).
Soil development is an important factor affecting water movement through soil.
McDonald (2002) classifies soil development in terms of surfaee age and pereent silt and
elay. Stronger soil development inereases water loss due to evaporation beeause a more
developed soil has higher amounts of silt and elay, whieh inereases soil water retention,
thus inereasing potential for evaporation of more water close to the surface (McDonald,
2002). Differences in soil development lead to differences in soil properties, resulting in
part from their difference in surface age. For example, abrupt changes in vegetation
cover at different points along a transect as observed in the Mojave Desert (Barbour and
Diaz, 1973) may be related to diseontinuities in depositional surfaee age (MeAuliffe,
1994; Parker, 1995).
Another important factor indicating soil age is the amount of cementation
between soil partiele fraetions, for example, by earbonate. Vesieles are observed to be
stabilized by soil eementing agents sueh as CaCOa (Evenari et al., 1974) and elays
(Sullivan and Koppi, 1991). Evenari et al. (1974) propose that, rather than direetly
111
causing vesicle formation, the presence of carbonate may help to cement the soil and
stabilize vesicles once they have formed. Carbonate enters the soil profile via
precipitation from solution (Evenari et al., 1974), from parent material and eolian dust,
and through the replacement of aluminosilicate grains by CaCOa (Reheis et al., 1992). In
addition to carbonate, several other compounds may be contributing to the cementation
and stabilization of vesicular pores. Under certain conditions, carbonate may be replaced
by silica (Arakel et al., 1989). Conditions in which pCOz values and groundwater pH
fluctuations correspond with periods of évapotranspiration, along with an increase in
elemental concentrations that are conducive to carbonate precipitation may also promote
the precipitation of silica in the soil moisture zone (Peterson and von der Borch, 1965;
Smale 1973; Carlisle et al., 1978; Arakel et al., 1989), leading to the occurrence of
silicified calcrete (Arakel et al., 1989). In arid environments, silica may also be present
in dust in the form of biogenic opal (Clarke, 2002). Additionally, clay and silt coatings
may provide vesicle stability during repeated wet and dry cycles (Sullivan and Koppi,
1991). Iron and aluminum oxide films coat and cement soil aggregates, preventing their
ready breakdown (Brady and Weil, 2004). It has been suggested that poorly crystalline
iron (hydr)oxides are better able to form soil aggregates than crystalline iron (hydr)oxides
because the former have a larger and more reactive surface area (Duiker et al., 2003).
The amount of organic matter in a soil may also contribute to soil stability; soil with
higher amounts of organic matter maintain better aggregate stability when wet than soil
with less organic matter (Brady and Weil, 2004). However, if iron is present, higher
amounts of organic matter may be detrimental to soil cementation. Analysis of poorly
crystalline iron (hydr)oxides in the more organic rich A horizons and more clay rich B
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horizons of two different soils indicates that iron is more important in forming
macroaggregates with clays when less organic matter is present in the soil (Duiker et al.,
2003). Another important cementing agent, particularly in arid environments, is salt.
Salt movement in arid environments is influenced by topography; salt moves through the
water table from areas of higher elevation to areas of lower elevation and from wetter
areas to drier areas (Brady and Weil, 2004). When the water evaporates, the salt remains
to accumulate in the soil (Brady and Weil, 2004).
Implications for this Study
Based on the parameters defining higher levels of vesicular horizon development
as defined in this study, the most strongly developed soil was found at Cottonwood Cove,
which also has the highest silt content of all three sites, as well as approximately 100%
clast cover. These latter two conditions should therefore increase soil water retention in
the uppermost horizon, and thus both evaporation and transpiration rates from the soil.
During the study year, from May 2004 to May 2005, rainfall data indicate that
Cottonwood Cove received the lowest amount of moisture out of all three sites, in both
the number of days of recorded rainfall and in its highest amount of precipitation
recorded for the year (0.5 in). Cottonwood Cove also had the deepest undisturbed
vesicular horizons of all three sites, and the best developed vesicular pores. Conversely,
the least strongly developed soil should be Airport Flat, which has a high sand content,
the lowest number of vesicular pores, and the lowest percent clast cover (approximately
90%) of all three sites. Airport Flat and Callville Wash received nearly the same amount
of rainfall throughout the study year, both in number of days that it rained and in the
113
amount of precipitation. Additionally, Airport Flat had the highest recorded number of
days of rainfall, and the second highest amount of rain recorded (1.93 in) after Callville
Wash (2 in), which both occurred on the same day, December 29, 2004. These data
indicate that the site with the lowest amount of rainfall during the study year.
Cottonwood Cove, also had the largest and most rounded pores, and therefore the best
developed vesicular horizons, according to this study, of all three sample sites.
The results presented for this study, paired with the influences on arid soil
processes discussed in this appendix, indicate the effect that these individual factors can
have even on the micro scale. Although these three sites are all within close proximity to
each other, many of their soil characteristics are very different from each other.
Cottonwood Cove, which had the lowest rainfall during the study year, one of the highest
percent clast cover, and the lowest chlorophyll content of all three sites, also has the best
developed vesicular pores, as defined by this study. The other two sites, Callville Wash
and Airport Flat, which both received higher amounts of rainfall throughout the year, also
have higher chlorophyll contents and less developed vesicular horizons, in terms of pore
shape and size, than Cottonwood Cove. These results may indicate an important
influence in the amount of rainfall on vesicular pore morphology, as well as overall
vesicular horizon development.
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VITA
Graduate College University of Nevada, Las Vegas
Maureen L. Yonovitz
Home Address:2105 Ridge Ave.Ann Arbor, MI 48104
Degrees:Baehelor of Arts, English and Geology, 2005 Hope College
Thesis Title: Pedogenesis of Vesieular Horizons in Disturbed and Undisturbed Soils
Thesis Examination Committee:Chairperson, Dr. Patriek Drohan, Ph.D.Committee Member, Dr. Matthew Laehniet, Ph.D.Committee Member, Dr. Miehael Wells, Ph.D.Graduate Faeulty Representative, Dr. Brian Hedlund, Ph.D.
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