Plant-cover effects on hydrology and pedogenesis in a sandy vadose zone

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Geoderma 118 (2004) 63–76

Plant-cover effects on hydrology and pedogenesis in a sandy

vadose zone

Rachel O’Brien*, C. Kent Keller1, Daniel M. Strobridge2

Department of Geology, Washington State University, PO Box 642812, Pullman, WA 99164-2812, USA

Received 24 July 2002; accepted 25 April 2003

Abstract

The hydrology of three field lysimeters was monitored after 15 years of plant growth to examine pedogenic impacts of plant-

induced changes to water budgets, flow, and chemical denudation. Between 70% and 87% of annual water discharge from all

lysimeters (‘‘sandboxes’’) was generated from short (usually V 5 days) precipitation events and longer (usually V 25 days)

snowmelt events. These infiltration events occur only 10–23% of the time throughout the year. Average annual

evapotranspiration (ET) from the red pine sandbox was 73% of precipitation and 1.4–2.6 times larger than ET from the

grass and moss/lichen boxes, respectively. In situ unsaturated hydraulic conductivity estimates, based on Darcy’s law, are on the

order of 10� 8 m/s and did not appear to vary significantly with plant cover. The upper zone of each sandbox is characterized by

rapid, large changes in the magnitude of pressure head (� 700 to � 6 cm), water content (1 to 9%), and magnitude and

direction of the hydraulic gradient. The vertical extent of this upper zone varies with plant cover; it extends to a depth between

30 and 70 cm beneath moss/lichen and grass whereas the upper zone beneath red pine extends to at least 120 cm during the

growing season. Synoptic field measurements of water content and pressure head were very different from laboratory

measurements of hanging-column moisture-retention curves. The red pine sandbox consistently exhibited the smallest water

contents and the most negative pressure heads. Upper-zone soil water samples, collected at small and large tension during

events, had different silica concentrations, which supports the existence of a dynamic flow system consisting of water residing

in large and small pore spaces, respectively. However, large-tension silica concentrations increased systematically with depth in

all sandboxes, indicating chemical reaction progresses in bulk soil water along flow paths. A conservative estimate of the

shortest mean soil water residence time in the sandboxes is 9 days, which is sufficient time for exchange and weathering

reactions to occur. Silica concentrations at 95 cm beneath red pine were twice as large as those beneath the moss/lichen cover.

However, larger water fluxes from the moss/lichen sandbox produced an annual silica denudation flux of 910 mol Si/ha/year,

whereas silica denudation from the pine sandbox was only 470 mol Si/ha/year. These results suggest that water and solute

uptake by rooting vegetation may actually decrease chemical denudation from young, soil-building ecosystems.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Unsaturated flow; Chemical denudation; Soil solution chemistry; Lysimeters; Soil development

0016-7061/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0016-7061(03)00184-8

* Corresponding author. Present Address: Department of Geology, Allegheny College, 520 N. Main St., Meadville, PA 16335, USA. Fax:

+1-814-332-2981.

E-mail addresses: [email protected] (R. O’Brien), [email protected] (C.K. Keller).1 Fax: +1-509-335-7816.2 Now at: Roy F. Weston, 1400 Weston Way, West Chester, PA 19380. (Fax: +1-610-701-7401).

R. O’Brien et al. / Geoderma 118 (2004) 63–7664

1. Introduction

Soil development can be rapid in certain ecosys-

tems, with diagnostic horizons forming in centuries to

milennia (see reviews in Birkeland, 1997; Schlesinger,

1997). Podzolization rates of sandy sediments (e.g.

aeolian dunes, fluvial bars, glacial tills/outwash and

marine beach deposits) in temperate and boreal cli-

mates have also been measured on these time scales

(Ugolini, 1968; see review in Lichter, 1998).

Investigations of soil development on decadal time

scales are far less prevalent in the literature, presum-

ably due to the fact that it is difficult to measure

changes to the solid phase over such short time

intervals. Plant-induced changes in both chemical

and physical soil properties have been observed within

decades in field lysimeters filled with a loam substrate

(Graham and Wood, 1991; Graham et al., 1995; Ulery

et al., 1995). In contrast, investigations of 30–40-year-

old red pine stands developed on sand document

changes to soil chemistry that were indicative of

pedogenesis, but no measurable changes in particle

size or other physical soil properties were observed

(Quideau and Bockheim, 1997; Quideau et al., 1996).

In sandy ecosystems, where physical changes in

soils are difficult to document over short ( < 100 year)

time scales, surface and/or groundwater mass fluxes

can provide an alternative means to investigate con-

trols on soil development and biogeochemical cycling

(Kellman and Roulet, 1990). This research takes

advantage of experimental field lysimeters (‘‘sand-

boxes’’) at the Hubbard Brook Experimental Forest

(HBEF). Our goal is to examine how water budgets,

hydraulic conditions, and the nature of unsaturated

groundwater flow vary beneath different aggrading

plant cover established on a sandy substrate. We posit

that a detailed examination of hydrologic character-

istics can help us understand how plants control the

distribution of water and solutes, and drive soil

development at the onset of ecosystem development.

The HBEF sandboxes were established in 1983–

1984 (Bormann et al., 1987) for the purpose of studying

nitrogen cycling and fixation (Bormann et al., 1993).

More recently, these systems have been used to inves-

tigate weathering processes (Berner and Rao, 1997;

Berner et al., 1998; O’Brien et al., 1998) and biogeo-

chemical fluxes (Bormann et al., 1998). The HBEF

sandboxes are particularly suited to study plant effects

on biogeochemistry, as the primary experimental de-

sign variable is plant cover. In addition, subsurface

flow conditions are simplified by the uniform, sandy

substrate. Each sandbox is fully lined and plumbed to

capture water throughput, facilitating a mass balance

approach to calculate aqueous fluxes within these

systems.

The objectives of this study were to: (1) compare

the effects of 15 years of plant growth on the

hydrology of a sandy vadose zone as a function of

plant cover; and (2) identify plant-induced changes to

hydrology that have implications for chemical denu-

dation and pedogenesis. We studied two sandboxes

with vascular plant cover (bunch grass and red pine

trees) and compared them to a sandbox with a non-

vascular plant cover (moss and lichen). Our specific

tasks included: (1) measuring water discharge and

calculating annual water budgets for 2 consecutive

years; (2) determining hydraulic properties of the

sediment with depth; and (3) documenting subsurface

hydrologic conditions and porewater chemistry to

identify mechanisms of water flow in the sandboxes.

2. Materials and methods

2.1. Site description

The HBEF is located in the White Mountains of

north-central New Hampshire, USA. Eighteen sand-

boxes were constructed in 1982 within a low relief

field location (at f 240-m elevation) to provide

contained soil and plant systems for mass-balance

studies. Based on particle-size analysis using the

pipette method (Klute and Dirksen, 1986), average

grain-size distribution of the sand is 94% sand, 4.5%

silt, and 1.5% clay-sized particles. We studied the

three large sandboxes (7.5� 7.5� 1.5 m deep) that

are fully lined and instrumented to collect all water

and solute exports. Drainage is routed from each

sandbox to a tipping bucket that allows for volumetric

discharge measurements and volume-weighted com-

posite sample collection. Tipping buckets are housed

in a subsurface chamber with a floor drain that

prevents discharge water from freezing.

Each sandbox studied contains a different plant

cover. Two sandboxes contain vascular plants: one has

red pine (Pinus resinosa Ait.) while the other consists

Fig. 1. Schematic (a) map view and (b) cross section of sandbox

instrumentation.

R. O’Brien et al. / Geoderma 118 (2004) 63–76 65

of bunch grass (Panicum virgatum L. and Andropo-

gon scoparius). The third sandbox was not planted

and has developed a nonvascular plant cover over

time (f 70% of the surface area consists of Cladonia

cristatella and Polytricum spp., a lichen and moss,

respectively). Details of sandbox construction and

maintenance are reported by Bormann et al. (1987)

and Ingersoll et al. (1987).

2.2. Data collection

All hydrologic monitoring and sample collection

occurred over a 24-month period comprising the 1996

and 1997 water years (June 1, 1996–May 31, 1998).

The water year (WY) at HBEF is from June 1–May

31. This interval provides the highest correlation

between precipitation and stream flow (Likens and

Bormann, 1995). In addition, the beginning of the WY

roughly coincides with the start of annual plant

growth in this area. Four hydrologic properties were

measured routinely during the study period: precipi-

tation amount, sandbox discharge, porewater pressure

head, and volumetric water content.

Precipitation amount was measured at HBEFweath-

er station #22, located 150 m northwest of the study

area. Daily precipitation values were determined by

taking weekly readings from a standard rain gage and

pro-rating them using hourly totals from a spring-

balance weight-recording gage connected to a chart

recorder (Federer et al., 1990). Sandbox discharge was

measured daily with the tipping buckets connected to a

data logger.

A vertical nest of four mercury-water-manometer

tensiometers (10, 30, 70, and 120 cm below ground

surface (bgs)) was used to measure porewater pressure

head in each sandbox. The spatial arrangement of

tensiometers, along with other sandbox instrumenta-

tion, is shown in Fig. 1. Water in the tensiometers was

drained between November and May of each year to

prevent freezing.

Volumetric water content was measured at 30-, 50-,

and 70-cm bgs on a weekly basis from June to Decem-

ber ofWY 1996 and from June to October ofWY 1997.

Water content measurements were not made during

winter and spring, when a snowpack was present,

because it was not possible to get to the neutron probe

access tube without compressing the sandbox snow-

pack. All measurements were made using a TroxlerR

4300 Neutron Probe, calibrated in a 0.8-m3 barrel filled

with the same sand used in the boxes at three different

moisture conditions (oven dry sand, saturated, and

field-capacity). Aluminum neutron access tubes (5.1-

cm diameter) were installed in the northern and south-

ern regions of each sandbox to a depth of 85-cm bgs.

Moisture release curves and saturated hydraulic

conductivity (Ksat) were measured on 5.4-cm diame-

ter� 2.54-cm high core samples taken from the sand-

boxes (Strobridge, 1998). Samples were collected

within 3 m of the tensiometer nest and northern

neutron access tube (Fig. 1a). Ksat was measured in

the lab using constant-head permeameters and follow-

ing the method described by Klute and Dirksen

(1986). Moisture release curves were determined by

the hanging column method, in which water outflow


from a sample is measured in response to each of a

series of pressure-head increments (Reginato and van

Bavel, 1962). Field-based, constant head Ksat meas-

urements were made in several locations in each of the

sandboxes using a 10-cm diameter infiltrometer ring

that was seated within the upper 5 cm of mineral soil.

A 3.5-cm water depth was maintained in the ring for 5

min to saturate the material and establish steady flow

conditions near the surface.

Porewater samples from the vadose zone were

collected from arrays of vacuum samplers installed at

three depths—15-, 35-, and 95-cm bgs—within each

sandbox. The samplers consist of a circular fritted

glass plate atop a glass collection chamber connected

to a vacuum pump. Each array consists of a fine-fritted

sampler that collects water under a small pressure head

(� 255-cm water matrix potential, or 255-cm water

tension) positioned adjacent to a coarse-fritted sampler

that collects water under a larger pressure head (� 10-

cm water matrix potential, or 10-cm water tension).

The chambers of these two samplers were cemented

with epoxy to a 25-cm-long sheet of galvanized steel.

Three such arrays were installed at each depth in

boreholes that were laterally hand-augered from a

temporary trench at the side of each sandbox. Arrays

were remotely placed against undisturbed material at

the top of the boreholes and anchored in place using

steel springs and grapples. Sampling arrays were

Fig. 2. Monthly discharge and precipitation values meas

installed at distances of 58, 108, and 168 cm from

the sandbox edge (Fig. 1a). Insulated subsurface sam-

pling access was installed in the trenches, which were

backfilled immediately following sampler installation.

All samplers were installed in 1995 and allowed to

equilibrate with the subsurface for 9 months prior to

sampling. A fourth sandbox, containing no topsoil or

plant cover (hereafter referred to as the ‘‘sand’’ box),

was also instrumented to collect porewater; dissolved

silica data from this box are reported here.

Porewater samples were analyzed for dissolved

silica by the colorimetric method of Strickland and

Parsons (1968). The absorbance of reacted samples

and standards was measured on a Milton Roy Spec-

tronic 601 at a wavelength of 810 nm; analytical error

associated with this technique was F 2%.

3. Results

3.1. Sandbox discharge

Monthly discharge from sandboxes responds to

snowmelt (usually January–April) and large rain

events (usually May–December) (Fig. 2). Sandbox

discharge patterns are similar to monthly stream dis-

charge from HBEF watersheds (data not shown).

Although the seasonal discharge pattern is similar

ured for the study period (June 1996–May 1998).

Fig. 3. Daily sandbox discharge and precipitation measurements during the autumn of WY 1996. (NV= nonvascular, G = grass, RP= red pine).


among the sandboxes, the magnitude of monthly

discharge systematically varies according to vegetative

cover: nonvascular>grass>pine. Annual precipitation

for WY 1996 and 1997 was 1441 and 1086 mm,

respectively; these values fall within F 20% of the

30-year HBEF average of 1195 mm.

Sandbox discharge is flashy, responding rapidly to

infiltration events and fluctuating by as much as a

factor of 30 in 1 day (Fig. 3); this pattern is repeated

in all sandboxes throughout the entire water year.

Differences in the magnitude of daily discharge

change occur among the sandboxes according to the

same pattern observed in monthly discharge (non-

vascular>grass>pine). Lag time between infiltration

and discharge is shortest for the nonvascular and

grass sandboxes and longer for the pine sandbox,

Table 1

Distribution of annual water discharge between baseflowa and eventb con

Plant cover Water year 1996

Number of

Qb days

Qb

(mm)

Qe

(mm)

Qe/

(%

Nonvascular 253 129 879 87

Grass 296 134 537 80

Red Pine 294c 74 394 84

a Qb: discharge during baseflow conditions (Q< 2 mm/day).b Qe: discharge during event conditions (Qz 2 mm/day).c Includes 86 days of no flow during WY 1996 and 176 days of no fl

especially during summer and autumn. The pine

sandbox typically had no discharge for most of the

growing season (Fig. 2); the onset of discharge during

the autumn of WY 1996 did not occur until the third

week of October (Fig. 3).

Annual sandbox discharge is separated into small-

flow (baseflow) and large-flow (event) components

(Table 1). Baseflow, defined here as water discharge

< 2 mm/day, occurs from each sandbox during 69–

88% of the water year. Events, defined as periods of

water discharge z 2 mm/day, only occur during 10–

23% of the year but produce between 70% and 87% of

the total water export from the boxes (Table 1). Thus,

flashy discharge in response to infiltration events is the

primary mode of water export from all three of the

sandboxes.

ditions

Water year 1997

Qt

)

Number of

Qb days

Qb

(mm)

Qe

(mm)

Qe/Qt

(%)

266 141 666 82

310 166 390 70

323c 51 182 79

ow during WY 1997.

Fig. 4. A comparison of discharge versus duration for every discharge event (z 2 mm/day) during the study period. Solid symbols indicate

discharge in response to rain events, while open symbols indicate a response to snowmelt events. Horizontal dashed line represents 1 pore

volume (the amount of infiltration needed to completely replace sandbox porewater).


Approximately 100 mm of infiltration is needed to

completely replace existing porewater in the sand-

boxes, i.e. one sandbox pore volume is about 100

mm (O’Brien, 2000). A plot of discharge magnitude

versus event duration (Fig. 4) indicates that only six

events exceeded 1 pore volume and could have com-

pletely replaced sandbox porewater; all of these oc-

curred during periods of snowmelt. More than half of

the total observed events involved less than 50 mm of

discharge. Provided flow is not predominantly via

preferential pathways (see Section 4.2), most sandbox

discharge is comprised of water that has resided in the

subsurface for some period of time—it is not infiltrat-

ing water from a current precipitation event.

3.2. Water budgets

Sandbox water budgets were estimated using a

simple water balance:

Precipitation ðPÞ � Discharge ðQÞ

� Evapotranspiration ðETÞ

¼ Change in soil moisture ðDSMÞ ð1Þ

Surface runoff from these systems is negligible, be-

cause of a level ground surface and large soil infiltra-

tion capacity (infiltration rates, based on the surface

Ksat measurements in Table 2, are on the order of 2000

mm/h, but precipitation intensities during the study

period never exceeded 17 mm/h). Given that annual

DSM is negligible relative to total water throughput,

ET is calculated as the difference between incident P

and sandbox Q.

Evapotranspiration as a percentage of annual pre-

cipitation averaged 28% for nonvascular, 51% for

grass, and 73% for pine; the pine percentage is larger

than the long-term mean of 40% reported for the

HBEF watersheds (Likens and Bormann, 1995).

Larger water uptake beneath an aggrading pine cover,

relative to a grass cover, is consistent with the

amount and distribution of roots within these sand-

boxes. Total red pine root mass concentration mea-

sured in 1988 was 17 Mg/ha (Bormann et al., 1993).

It is reasonable to assume pine roots extended to the

bottom of the sandbox; lateral coring during installa-

tion of the porewater samplers in 1997 exposed large

(>2 mm) pine roots at 35- and 95-cm depth. In

contrast, roots were not visible at either of these

depths in grass sandbox cores. Roots in the grass

Table 2

Hydraulic conductivity estimates (m/s) as a function of plant cover

Nonvascular Grass Red Pine

0–10 cm 70–120 cm 0–10 cm 70–120 cm 0–10 cm 70–120 cm

Ksat fielda 6.8� 10� 4

[8, 0.05]

– 7.8� 10� 4

[10, 0.20]

– 4.1�10� 4

[5, 0.22]

–

Ksat labb 2.1�10� 4

[16, 0.26]

– 3.7� 10� 4

[14, 0.09]

– 3.1�10� 4

[8, 0.07]

–

Kunsatc – 2.2� 10� 8

[� 14]

– 1.9� 10� 9

[� 26]

– 2.4� 10� 8

[� 27]

a Mean value of constant-head field infiltration tests conducted at the sediment surface (Strobridge, 1998). Values in brackets are the number

of measurements and coefficient of variation for the measurements (S.D./mean), respectively. Value reported for red pine was measured on the

mineral soil; measured conductivity of red pine litter layer was 1.1�10� 3 m/s (6, 0.05).b Mean value determined from constant head permeameter experiments on undisturbed cores (Strobridge, 1998). Values in brackets are the

number of measurements and coefficient of variation for the measurements (S.D./mean), respectively. Values reported for red pine were

measured on core samples collected from an adjacent pitch pine sandbox.c Darcian calculations of unsaturated hydraulic conductivity. Porewater flux was measured as sandbox discharge when unit hydraulic

gradient conditions existed in the field, based on porewater pressure head values at 70- and 120-cm depth. Value in brackets is the mean pressure

head value (in cm water) measured at depth during unit gradient conditions.


sandbox are assumed to be concentrated within the

upper f 20 cm of the subsurface, which is consistent

with global data for temperate grasslands (Jackson et

al., 1996). Grass root mass as of 1997 was assumed

to be 5 Mg/ha based on data reported by Marschner

(1995).

Fig. 5. Synoptic field measurements of porewater pressure head and volum

For purposes of clarity, only data collected at 30-cm depth are shown. Lab

the grass sandbox using the hanging water column method (Strobridge, 19

legend. Vertical line shows the limit of pressure head values above which

3.3. Porewater pressure head and water content

At 30 and 70 cm beneath all three sandboxes,

field-measured volumetric water content ranged from

1% to 9% and pressure head ranged from � 700 to

� 6 cm of water (Fig. 5). The pine box consistently

etric moisture content during the growing season (June–October).

oratory water release curves were measured on sediment cores from

98). The midpoint depth of each core sample is noted on the figure

macropores z 0.5 mm in diameter can fill with water.


exhibited the smallest pressure head (values at all

depths were consistently between � 600 and � 300

cm of water throughout the growing season) and

smallest water content. Water content and pressure

head values for the grass and nonvascular sandboxes

were very similar at 30 cm (Fig. 5), suggesting that

significant water uptake via grass roots does not occur

at this depth. Increases in sandbox pressure head

correspond closely in time to measured increases in

water content (Fig. 6). This pattern was observed

beneath all plant covers (Strobridge, 1998 reports the

entire data set).

3.4. Hydraulic conductivity

For a given sandbox, the difference between field-

and lab-measured saturated hydraulic conductivities

(Ksat) is within a factor of 4, with field values

consistently larger than lab-measured values (Table

2). This is a relatively small range of difference for a

physical property that can vary over 13 orders of

magnitude (Davis, 1969). Bulk field unsaturated hy-

draulic conductivity (Kunsat) at small tensions (� 30

to � 10 cm water) is four to five orders of magnitude

less than the measured Ksat values (Table 2), which is

Fig. 6. Hydrologic measurements for the grass sandbox (WY 1997 growing

measured at 30-cm depth.

a typical decrease for sandy soils (e.g. Michiels et al.,

1989; Marshall et al., 1996, Hillel, 1998).

3.5. Hydraulic head and the direction of water flow

Unsaturated groundwater flow occurs in response

to gradients of total head (also termed hydraulic head).

In the absence of osmotic forces, total head (HT) can

be given as the sum of pressure head (HP) and

elevation head (HZ):

HT ¼ HP þ HZ ð2Þ

Depth profiles of total head indicate the magnitude

and direction of hydraulic gradients and unsaturated

flow within each sandbox over time (Fig. 7a,b). We

present data only for grass and red pine sandboxes;

nonvascular sandbox data are very similar to grass

sandbox data.

Rapid changes in hydraulic head and gradients

observed in the sandboxes (Fig. 7a,b) are consistent

with rapid water infiltration. Patterns in the grass

sandbox indicate rapid changes in hydraulic head

over the 10–70-cm depth interval, whereas steady,

small values of hydraulic head are observed at 120

season). Porewater pressure head and volumetric water content were

Fig. 7. Time series of total (hydraulic) head measurements at several depths for (a) grass and (b) red pine sandbox during the WY 1997 growing

season. Note the change in ordinate scale for the pine sandbox.


cm (Fig. 7a). Water flow between 70 and 120 cm in

the grass sandbox was vertically downward for most

of the growing season (Fig. 7a). In contrast, the pine

sandbox exhibits no steady values of hydraulic head

at depth (Fig. 7b). Predominantly upward fluxes

occur between 70 and 120 cm, presumably due to

removal of water at depth by roots. Upward fluxes,

coupled with smaller water contents at depth beneath


the pine cover, are consistent with the absence of

discharge from this sandbox during most of the

growing season.

3.6. Silica concentrations and denudation fluxes

Profiles of porewater silica concentration (Fig. 8)

illustrate that porewater concentrations increase with

depth in all sandboxes. Although the magnitude of

specific sample concentrations varies with plant cover

and by season, the pattern of increasing solute con-

centrations with depth occurs throughout the entire

water year. The silica concentration at 95 cm for red

pine sandbox is approximately twice as large as

measured concentrations beneath other plant covers

(Fig. 8).

Even though concentrations are largest at depth

beneath pine cover, total mass export in solution, i.e.

chemical denudation, is largest beneath nonvascular

cover. Chemical denudation is calculated as the prod-

uct of water discharge and the concentration of a

dissolved species of interest in that discharge,

summed over a period of time. For water year 1997,

silica denudation flux from the nonvascular sandbox

Fig. 8. Mean silica concentration profiles for each sandbox. Concentrations

samples were collected at � 255 cm of pressure head. Samples collected

was 910 mol Si/ha/year, while the red pine denudation

flux was only 470 mol Si/ha/year.

4. Discussion

4.1. Zones of water distribution and movement

Two distinct hydraulic zones occur in the sand-

boxes. The ‘‘upper zone’’ is characterized by rapid,

large changes in hydraulic head (90–800 cm) that

occur in response to episodic infiltration events. These

changes in head are concomitant with the filling of

larger pores within the sand matrix for a short period.

The filling of large pores during episodic infiltration

creates conditions consisting of more rapid flow in

large pores and slower flow in smaller pores. Disper-

sive mixing of this water should occur as water

advects through the upper zone.

In contrast, the ‘‘lower zone’’ is characterized by

more gradual, smaller changes in hydraulic head (5–50

cm) where water content values are relatively constant

and porewater moves in a narrow range of pore sizes.

Relative to the upper zone, groundwater flow should be

plotted below the dotted line represent discharge samples. Porewater

on May 25, 1997.


less dynamic and less dispersive in the lower zone. The

vertical extent of each of these zones in the sandboxes

varies with plant cover. For the nonvascular and grass

sandboxes, the upper zone extends to a depth between

30 and 70 cm (Fig. 7a). The red pine box, however,

exhibits an upper zone that extends to at least 120 cm

during the growing season (Fig. 7b).

Porewater silica concentration data support the

existence of a range of porewater velocities and

residence times within the upper zone. Large infiltra-

tion events in the upper zone occasionally permit the

extraction of water held under small tension (large,

but still negative, values of pressure head). Small-

tension water, which resides in larger pores, consis-

tently had smaller silica concentrations than water

residing in smaller pores that was synoptically col-

lected under larger (more negative) tension (Fig. 9).

These concentration differences decline for a given

sampling location and depth as subsurface water

content increases at the end of the growing season

(note trajectories for a particular sampler from late

summer into autumn in Fig. 9). Concentration differ-

ences in porewater beneath both a nonvascular plant

Fig. 9. Porewater silica concentrations for synoptic water samples collected

symbol) represents a unique small/large pressure-head sampler array monit

indicated next to each data point.

cover and the bare-sand box (Fig. 9) indicate that

water mixing in the upper zone is not solely the result

of plant root activity.

Our inability to extract small-tension porewater

from the nonvascular and grass sandboxes at 95-cm

depth further supports the idea that water in the lower

zone resides in a smaller range of pore sizes and

moves at a more uniform rate. Variations in the flow

field that may occur within the upper zone are

dissipated within the lower zone.

4.2. Preferential flow?

Rapid increases in sandbox discharge could be

interpreted as the result of preferential flow during

infiltration; fingered flow has been observed in coarse-

textured soils (Hill and Parlange, 1972; Glass et al.,

1989). However, topsoil that was tilled into the upper

20 cm of the sand helps retain moisture and provides

some structure to the soil. Water content data indicate

that even shallow sand remains moist in the nonvas-

cular and grass sandboxes, which would reduce the

development of flow fingers (Selker et al., 1999).

at different pressure heads. Each data series (denoted by a different

ored through time (details in Section 2.2). The month of sampling is


The tensiometers we have used in this work

respond to pressure-head changes in liter-sized vol-

umes of sand, and the neutron probe detects water

content in similar volumes. Our instruments thus

monitor hydraulic head changes in bulk sand. Such

changes in the upper zone precede or nearly coincide

with discharge increases (Fig. 6), which is consistent

with flow through the bulk of a very conductive

unsaturated sand matrix (Table 2) in the lower zone

of the sandboxes. Discharge from these systems,

therefore, represents lower-zone porewater that is

hydraulically ‘‘pushed’’ out by water above via

piston flow. Water flow through bulk matrix of the

sand is supported by two additional pieces of evi-

dence: (1) our inability to extract small-tension pore-

water from any of the sampler arrays in the lower

zones, even during events, and (2) measured

increases in porewater silica concentration with depth

(Fig. 8).

4.3. Implications of porewater flow on chemical

denudation and soil development

A conservative estimate of the shortest mean pore-

water residence time for any of the sandboxes is 9

days. This is more than enough time for cation

exchange (Sposito, 1989) and mineral dissolution

reactions (Figs. 8 and 9; White, 1995) to occur. These

sandy vadose zones appear to support water–mineral

contact with enough time for substantial mass transfer

between solid and aqueous phases. These vadose

zones also provide for relatively rapid aqueous mass

transport out of the sandboxes during infiltration

events. The combination of these conditions yields

large rates of chemical denudation.

Chemical denudation removes mineral mass from

landscapes, lowering them over geologic time. By

assuming weathering of sandbox plagioclase (An23,

Bormann et al., 1998) and a constant soil bulk density

(1.6 g/cm3) during denudation, we can convert the

nonvascular denudation flux to a crude rate of land-

scape lowering; the result is 5.5 cm/10,000 years. The

smaller red pine rate of 2.9 cm/10,000 years, obtained

using the same assumptions, suggests that landscape

lowering might slow as vegetation and soil develop-

ment progress. A more accurate estimate of landscape

lowering is difficult to measure (e.g. Vitousek et al.,

1997) and would require further investigation.

The red pine and nonvascular sandboxes illustrate

the difference between biologically and hydrologically

driven soil development. In the red pine sandbox,

hardy vascular plants have become established and

grown for over a decade. Large chemical weathering

fluxes, to satisfy the nutritional demands of growing

trees, support aggradation of organic matter and drive

the precipitation of secondary minerals (Bormann et

al., 1998; Keller et al., 1999). With time, these pro-

cesses reduce the mean grain and pore size of the soil,

thereby improving water retention in the vadose zone.

Biologic uptake of water and nutrients controls and

reduces hydrologic export and chemical denudation

from the system; nutrient mass is retained within the

soil and a pronounced vertical soil profile develops.

In the hydrologically driven scenario, represented

by the nonvascular sandbox, lack of plant growth

translates to smaller chemical weathering fluxes and

no organic matter aggradation (Bormann et al., 1998).

In the absence of plants, water throughputs in the sand

are large and so the chemical denudation is also large.

A global relationship between chemical denudation

and water flux has been observed at the watershed

scale (Holland, 1978; Alexander, 1988). Prosser and

Roseby (1995) report rapid leaching of solutes from

the soil profile for the first 5 years after revegetation

of disturbed sandy material.

In a temperate climate with acidic precipitation in

excess of 1000 mm/year, why is not significant

elluviation/illuviation occurring? Circum-neutral pH

values at all depths in the sandboxes (due to cation

exchange and mineral dissolution) prevent vertical

translocation and deposition of iron and aluminum

to depth in the profile (O’Brien and Keller, 2000) and

carbonate minerals have been leached from the sand.

Under these particular geochemical conditions, water

fluxes are exporting plant nutrients from the system

and the textural changes that serve to retain water and

solutes are slow to develop.

4.4. Field lysimeters as a proxy for ecosystems

A mass-balance study of 240 ka of silica loss and

soil development on sands in a temperate climate

reports an average silica denudation rate of 745 mol/

ha/year (Chadwick et al., 1990). Denudation rates

from the sandboxes (470–910 mol/ha/year) bracket

this value. Mean annual evapotranspiration for the red


pine sandbox (73% of precipitation) is remarkably

similar to a mean value of 75% reported in a temper-

ate lodgepole pine forests (Knight et al., 1985). While

we recognize field lysimeters are simplified models of

complex natural systems, they may be a useful proxy

for investigating certain ecosystem processes.

5. Conclusions

Hydrologic data indicate the existence of two

distinct zones of unsaturated flow within the sand-

boxes: an upper mixing zone characterized by large

variations in flow rates and directions, and a lower

zone with much steadier downward flow. The thick-

ness of the upper zone ranges from < 70 cm to over

120 cm and varies with plant cover. Porewater silica

concentrations collected under large tension increase

with depth beneath all plant covers, supporting the

conception of lower-zone piston flow with increasing

reaction progress along flow paths. While silica con-

centrations beneath the red pine cover were approxi-

mately 2� larger than those beneath grass and

nonvascular cover, water discharge was much smaller

under pine. Thus, the silica denudation flux was

largest from the nonvascular sandbox (910 mol Si/

ha/year) whereas the flux from the red pine sandbox

was only 470 mol Si/ha/year. This suggests that water

and solute uptake by growing plants may decrease

chemical denudation while promoting soil develop-

ment in certain young ecosystems.

Acknowledgements

Support for this work came from NSF grant

EAR96-28296 and the A.W. Mellon Foundation. The

authors would like to thank: employees of the US

Forest Service, Institute of Ecosystem Studies, and the

sandbox working group for logistical and scientific

support; Vinnie Levasseur, Jason Demers and Adam

Welman for help with routine field measurements; and

Dr. Karen Von Damm at the University of New Hamp-

shire for the use of her lab for silica analyses. Dr. W.B.

Bowden at the University of Vermont installed the

tensiometers and neutron probe access tubes, and

provided us with hydrologic data collected between

1990 and 1991. Insightful comments provided by Drs.

B.T. Bormann, A. Dougill and A. McBratney consid-

erably improved this paper.

This is a contribution of the Hubbard Brook

Ecosystem Study and the program of the Institute of

Ecosystem Studies. Hubbard Brook Experimental

Forest is managed by the USDA Forest Service,

Northeastern Research Station, Newtown Square, PA.

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Documents

Plant-cover effects on hydrology and pedogenesis in a sandy vadose zone