Decomposition of Avena sterilis litter under arid conditions

Journal of Arid Environments (2000) 46: 281–293doi:10.1006/jare.2000.0672, available online at http://www.idealibrary.com on

Decomposition of Avena sterilis litter under aridconditions

Z. Hamadi*, Y. Steinberger*?, P. Kutiel-, H. Lavee- & G. Barness*

*Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52900, Israel-Department of Geography, Bar-Ilan University, Ramat Gan 52900, Israel

(Received 21 January 1999, accepted 29 June 2000)

The influence of abiotic conditions on Avena sterilis plant litter decompositionwas studied at three sites along a topoclimatic gradient in the Judean Desert,Israel. Decomposition of A. sterilis plant litter followed a two-phase pattern:early and rapid mass loss during the winter season followed by a long period oflow mass loss during the remaining seasons. Differences in decompositionrates were found between winter and summer, and between material on the soilsurface and buried in the soil. There were significant differences indecomposition rates attributable to topoclimatic location, to which the shortrainy-winter season contributed significantly. Lignin concentration was foundto increase significantly during the rapid phase of decomposition, howevera decrease in this value occurred during the remaining dry seasons. Litternitrogen content oscillated during the study period such that in the buried littera bi-phasic pattern similar to the litter decomposition in the above-ground litterwas obtained. These results suggest that litter decomposition is not affec-ted by climate but by litter lignin content, since an inverse relationship wasfound between lignin content and the decomposition rate.

( 2000 Academic Press

Keywords: litter; decomposition; lignin; desert; topoclimatic gradient

Introduction

The biotic components in terrestrial ecosystems are mainly regulated by climate andorganic matter content (Slapokas, 1991). Because primary production is stronglycorrelated with the availability of nutrients, the decomposition and mineralization ratesof dead plant material is of great importance. Despite the importance of such processes,most research studies have been conducted in humid ecosystems, where micro-organ-ism biomass and nutrient availability are high (Melillo et al., 1982; Anderson et al., 1985;Hunt et al., 1988; Cochran, 1990; Taylor et al., 1991). However, some decompositionstudies have been performed in arid and semi-arid ecosystems with extreme ranges ofmoisture availability and many other climatic factors, mostly in North American deserts(Elkins & Whitford, 1982; Parker et al., 1984; Schaefer et al., 1985; Whitford et al.,1988), with only a few studies in the Negev Desert of Israel (Steinberger & Whitford,1988; Steinberger et al., 1990). The North American studies, especially those conducted

?Address for correspondence: Prof. Y. Steinberger, Faculty of Life Sciences, Bar-Ilan University, RamatGan 52900, Israel. Tel: 972-3-5318571; Fax: 972-3-5351824; E-mail: [email protected].

0140-1963/00/110281#13 $35.00/0 ( 2000 Academic Press

282 Z. HAMADI ET AL.

in the Chihuahuan Desert, have elucidated several surprising features of the decomposi-tion rate in deserts: (1) the rate of organic matter decomposition is high and almost equalto that reported for the wet tropics (Schaefer et al., 1985); (2) the rate of decompositionis not correlated with lignin content or actual evapotranspiration (Whitford et al., 1981;Santos et al., 1984; Schaefer et al., 1985); (3) there is no relationship between the litterdecomposition rate and litter quality (Schaefer et al., 1985); and (4) soil fauna acceleratethe decomposition of buried litter but not surface litter (Parker et al., 1984). In addition,Berg (1984) and Offer et al. (1992) suggested the possibility that soil nutrients,especially nitrogen, may play an important role in controlling the litter decompositionrate.

It should be mentioned that the Chihuahuan Desert, where most of the studies wereconducted, is characterized by summer rainfall storms in addition to winter rainfalls,reaching a multi-annual average of 250 mm rainfall. In more extreme arid areas, onlya few studies have been undertaken in an attempt to isolate the abiotic components bycomparing different climate zones (Strojan et al., 1987; Steinberger et al., 1990).

In this study an attempt was made to evaluate the importance of climate in controllingdecomposition rates. Climate was focused upon by using identical plant litter as thesubstrate for decomposition, and by using a constant slope inclination and orientation ofthe experimental plots, where the climate varied by location plots along a topoclimaticgradient in the Judean Desert.

Study site

The Judean Desert is a rain shadow desert which occupies the east-facing slope of theJudean mountains. Its eastern border is the Dead Sea, which is about 390 m below sealevel. In the west the desert merges with the Jerusalem mountains, 600 m above sea level.The mean annual rainfall decreases from a multi-annual average of 650 mm in thewestern part to less than 100 mm in the east (Rosenan, 1970a, 1970b), with a greatvariability between years.

The study was conducted at three sites along a climatological gradient running fromthe Judean mountains toward the Dead Sea. The three sites, representing Mediterra-nean, mildly arid and arid conditions (Fig. 1), are as follows:

(1) Giv’ at Ye’arim (GIV)—this site is located 11 km west of Jerusalem at an elevationof 650 m above sea level. The average annual rainfall is 620 mm and the meanannual temperature 173C.

(2) Mishor Adumim (MIS)—this site is located 11 km east of Jerusalem, midwaybetween Jerusalem and the Dead Sea, at an elevation of 230 m above sea level.The average annual rainfall is 260 mm and the average annual temperature 203C.

(3) Kalia (KAL)—this site is located 4 km west of the Dead Sea, at an elevation of70 m below sea level. The average annual rainfall is 120 mm and the averageannual temperature 233C.

Hill slopes having a similar gradient (11–133) and aspect (azimut of 140–1503) werechosen at all three sites. The basic characteristics of the study sites are presented inTable 1. The bedrock at all sites is carbonatic. In spite of similar lithology andtopographical conditions, different soil types have developed in response to long-term effects of average climatic conditions during the last millennia. At the Mediter-ranean site (GIV), a red soil, Terra Rossa, has developed which contains a relatively highamount of clay (50%) and a small amount of calcium carbonate (6–8%). In the mildlyarid site (MIS), a non-saline light brown lithosol has developed. This lithosol isa shallow, carcareous loamy soil with 20% clay. At the arid site (KAL), a light salinedesert lithosol which contains 10% clay has developed.

Figure 1. Map of the area and the sampling sites.

DECOMPOSITION OF AVENA STERILIS 283

The vegetation composition also differs between the sites: the GIV site isdominated by Quercus calliprinos}Pinus halepensis shrubland, accompanied by otherMediterranean sclerophyll phanerophytes and chamaephytes such as Sarcopoteriumspinosum, Cistus salvifolius, Fumana arabica and F. thymifolid. At the MIS site,the vegetation is dominated by an Anabais articulata}Halogetan alopecuroides associ-ation, accompanied by Salsola vermiculata and Reaumuria hirtella. At KAL, thevegetation is composed of the Zygophyllum dumosum}Reaumuria hirtella association.One of the most common annual plants found along the topoclimatic gradient is Avenasterilis.

Table 1. The main geographical and climatic data of the four study sites along the transect

Slope angle Average Average& direction annual annual

Elevation relative (face) of the rainfall temperatureStudy site to sea level slope (mm) (3C)

Givat Ye’arim 650 m 133, S-facing 620 13Mishor Adumim 320 m 113, S-facing 260 17Kalia !60 m 123, S-facing 110 20


Materials and methods

Litter decomposition rate was studied using the litter bag technique and Avena sterilisplant material was collected from the field at the end of the growing season andoven-dried at 603C for 72 h. Preweighed, 10$0)1 g samples of litter were placed in20]20 cm (mesh"1)5 mm) bags made from fiberglass window screen. A total of 650bags were prepared for the first year of study (1991–1992), and a second set of 480 bagswas prepared for the second year (1992–1993). Each litter bag was placed in a plasticbag during transportation to and from the field in order to minimize loss of plantmaterial.

In November 1991 (t01

), a total of 180 litter bags were placed at each site with 120 onthe soil surface (secured with wire pins to prevent movement) and 60 buried in the soil ata 10 cm depth.

Ten of the surface litter bags and six of the buried bags were collected at each siteevery 45 days, along with duplicate soil samples from the 0–5 cm and 5–10 cm depths.

In October 1992 (t02

), the second set of 90 surface litter bags and 80 buried bags wasinstalled at each site. Eight litter bags (8 surface bags, 8 buried bags) were retrieved fromeach site at 60 day intervals.

Mass loss determination

Mass loss was determined for half of the litter bags (4 litter bags) retrieved from the field,whereas the remaining bags were used for lignin and total N content analyses.

Litter bags for mass-loss determination were first dried at 603C for a minimum of 72 h,weighed, and then placed in a muffle furnace at 4903C for 8 h. The organic matterweight loss was obtained using the Elkins & Whitford (1982) equation. The first orderkinetic constant (k) for mass loss was calculated as follows:

A5"A

0e~,5

where A5is the mass remaining at time t, A

0is the initial mass, t is time and k is a negative

coefficient (Swift et al., 1979).

Chemical analysis

Total plant nitrogen and lignin content were measured on dry plant tissue. Theremaining litter was Wiley milled to a size of 2 mm. Nitrogen and lignin content weredetermined using the micro-Kjedahl technique (Bremner & Mulvaney, 1982) and VanSoest’s (1963) methods, respectively.


Abiotic parameters

Percentage of soil moisture was determined gravimetrically for all soil samples by usinga 603C oven for a minimum of 72 h. Total soil organic matter was determined bymuffling dry soil samples at 4903C for 8 h. Soil temperature and rainfall wereobtained by using a CR10 data-logger (Campbell Scientific Inc., Logan, Utah). All datawere subjected to analysis of variance (ANOVA) and Duncan’s Multiple Range Test.

Results

Between November 1991 and September 1992, rainfall totalled 660, 260 and 110 mm atGIV, MIS and KAL, respectively. These amounts were found to be above or equal tothe multiannual averages at these locations (Fig. 2). The corresponding values in thesecond year (October 1992 to July 1993) were 548, 202 and 47 mm, and weresignificantly lower than those observed during the first year of the study. The variation inthe amount and distribution of rainfall caused soil moisture contents to vary at both the0–5 cm and 5–10 cm layers.

Significant differences (p(0)005) in soil moisture contents were found amongthe different sites between November 1991 and October 1992: the mean moisturewas 5)2% at KAL, 11)5% at MIS and 16)8% at GIV. In the second year (October 1992 toJuly 1993), no significant differences in moisture content were found between MISand KAL, the two more arid sites, due to relatively low rainfall and high evapotranspira-tion. However, a significant (p(0)05) difference in moisture content was found atthe 0–5 cm soil layer between those two stations and the GIV site.

Soil surface temperature pattern was found to be similar at all locations during the firststudy period, ranging from a minimum of 03C to a maximum of 603C (Fig. 2). Duringthe second year, the temperature oscillations were generally similar at the two more aridstations (MIS and KAL), whereas at GIV the relatively high temperature continued untilthe first significant rainfall storm.

Significant differences in soil organic matter (p(0)0001) were found among thedifferent stations along the transect, with mean values at the three stations being 4)3,7)7, and 8)1% at KAL, MIS, and GIV, respectively. Moreover, soil organic matter wasfound to differ significantly (p(0)0001) between winter and summer, whereasduring the winter period it was found to be higher than during the dry summer seasons.However, no significant differences (p(0)05) were found between the two layers(0–5 and 5–10 cm depths).

The litter weight loss ranged between 20 and 70% from the above and below litterbags, respectively, at the end of the rainy season (Fig. 3). Changes in weight loss werefound to be negligible during the dry summer period. No differences were foundalong the gradient in the rates of plant litter decomposition at the soil surface. However,during the first year, a significant increase in decomposition rates was found at the!10 cm layer (p(0)001) compared to the above-ground litter at all locations. Duringthe second year, a similar pattern was observed in litter decomposition placed atdifferent sites for both first and second litter bags at different periods (t

01and t

02).

A similar ratio of litter decomposition at !10 cm was observed along the transect atall three locations, with approximately 50% decomposition over a 1-year period.

The kinetic constants calculated from dry weight loss (Table 2) indicated that duringthe study period the highest rate of weight loss at all locations occurred during the rainyseasons of the two years. At the Kalia site, a relatively high k value was found comparedto the two less arid stations, which may indicate a relatively higher response of soil biotacomponents to low and high water input.

There were significant differences in decomposition rates attributable to thetopoclimatic location, to which the occurrence of the wet season contributed

Figure 2. Rainfall distribution, water content and soil surface temperature during the 1991–1992study period at different locations along the topoclimatic gradient.


significantly (p(0)05). However, during the dry season these differences disap-peared in both above and below surface litter bags along the topoclimatic gradient.

The initial chemical values for the A. sterilis litter were 45)0$1)5% lignin and8)3$1)1 mg g!1 nitrogen per dry weight. The litter nitrogen content oscillated duringthe study period such that during the wet season a decrease of 1)5% was observed froman average of 8)3 mg g!1 total N in the litter buried in soil, while during the dry season

Figure 3. Changes in percent mass loss of Avena sterilis plant litter, expressed as plant materialremaining at different locations along the topoclimatic gradient.


the total nitrogen stabilized in the buried litter. An opposite trend was found in N in thelitter placed on the soil surface, where a significant increase (p(0)05) in nitrogen wasobserved. It has been suggested that such an increase is due to the contribution of theaeolian dust storms (Offer et al., 1992).

The lignin concentration in the litter placed on the soil surface increased to approxi-mately 118, 113 and 137% at Giv’at Ye’arim, Mishor Adumim and Kalia, respectively,during the first winter period. After this period, which was followed by the dry season,

Table 2. Kinetic rate constants for dry weight loss at the different locationsalong the topoclimatic gradient

Giv’at Ye’arim Mishor Adumim, KaliaTime period (K 10!3 day!1) (K 10!3 day!1) (K 10!3 day!1)

Nov 1991–Apr 1992 4)5a 5)4a 3)6b

Apr 1992–Dec 1992 0)2d 0)2d 2)0c

Number followed by the same letter is not significantly different (p(0)05).


the lignin content at all locations decreased to levels ranging from 108 to 114% of theoriginal concentration.

The changes in the lignin concentration of buried A. sterilis litter at all locationsincreased to 120% of its original concentration during the winter season. After thisseason, at the end of the study period, lignin concentration increased and stabilized at109% of its original level.

Changes in the lignin/nitrogen ratio followed the wet and dry seasons (Table 3) withless consistency in the above- and below-ground litter placement. During the wet seasonthere was a general increase in the percentage of lignin and a general decrease in thepercentage of nitrogen, revealing a significant increase in the L/N ratio along thetransect. The significant differences (p(0)05) obtained between the stations aswell as between the litter location were found to be correlated to moisture availability asaffected by the rainfall.

Discussion

In the climatic gradient from an above sea level site with 650 mm rainfall and a multi-annual temperature of 13)53C, to an arid site located below sea level (!60 m) with anaverage rainfall of less than 100 mm, a significantly higher mean average temperature(203C) represents a powerful and important experimental tool for triggering bioticprocesses. Soil organic matter along this gradient was found to be similar to datareported by Fuller (1990), Knight (1990), Melillo et al. (1982) and Swift et al. (1979),who claimed that abiotic factors (e.g. soil moisture and temperature) regulate soilorganic matter. This resulted in a significant difference in organic matter at the twoextreme stations. Studies on above-ground biomass along the same transect in April1992 at the end of the rainy season demonstrated relatively high primary production atKAL compared to MIS. Such differences are related to the high quantities anddispersion of rainfall, causing multiple germination at the extreme station, Kalia.

According to Fuller (1990), soil organic matter acts as a carrier, fulfilling an importantrole in soluble nitrogen distribution. Therefore, an increase in nitrogen was expectedduring the high rates of organic matter decomposition (Wallace et al., 1978; Crawford& Gosz, 1982; Whitford, 1988; Fuller, 1990; Slapokas, 1991) at the different sites.However, the soluble nitrogen obtained was low and similar along the transect. Thisdecrease could be caused by leaching of nitrogen to deeper layers, or by use of primaryproducers. Our results differ from Cochran’s study (1990), which was conductedin an extreme cold climate. According to his working hypothesis, the decompositionrates above and below the soil surface are not a function of moisture content, but ratherof nitrogen availability, being consumed by biotic communities as an energy source.Since nitrogen availability in the litter was not found to be limited, we assumed thatnitrogen availability does not affect decomposition rates along the topoclimatictransect.

Table 3. Lignin/nitrogen ratio in the A. sterilis litter located above- and below- (10 cm) ground during the decomposition study period

Giv’at Ye’arim Mishor Adumim Kalia

10 cm 10 cm 10 cmDate Above-ground below-ground Above-ground below-ground Above-ground below-ground

10/92 51)9 51)9 51)9 51)9 51)9 51)912/92 60 78)6 73)4 51)4 70 82)702/93 58 54)2 69 49)4 56)3 70)104/93 38)1 22)8 46)9 35)2 91)9 45)406/93 49)5 28)6 53)1 33)5 36)4 N.D.*

*N.D.—no data available.

DE

CO

MP

OS

ITIO

NO

FA

VE

NA

ST

ER

ILIS

289


No significant differences were found in plant litter decomposition rates ofA. sterilis on the surface litter along the transect. Mass loss during the first months (60days) of 1991 took place in relatively dry weather. According to Swift et al. (1979) andSlapokas (1991), such weight loss occurs as a result of leaching, rain and maybe dew innew organic material added to soil. Tripathi & Singhi (1992) also found similar datawhile studying grass in India’s savannas, and Bell (1974) pointed to climatic conditionssuch as breezes, radiation, precipitation and soil humidity as factors regulating thedecomposition rate.

A. sterilis litter buried at a 10 cm depth exhibited a significantly higher decompositionrate than the litter placed on the soil surface. In both cases, with the surface and theburied litter, biphasic decomposition was observed, similar to data reported by Steinber-ger & Whitford (1988), with high rates during the winter rainy season and no significantmass loss during the dry season at all locations along the transect. Such results demon-strate the importance of moisture availability on soil biota activity (Fuller, 1990; Knight,1990). However, no significant correlation was observed between rainfall and thedecomposition rate. Similar results were reported by Eijsackers & Zehnder (1990) whoclaim that the high rates of lignin, which are not affected by humidity and dryness,prevent such dependence.

During the second year, mass loss and rates of mass loss were found to be similar tothe first year at GIV and MIS, whereas at KAL, as a result of the harsh winter climateunique to this year (snow events), which may have a strong effect on plant cuticulaallowing leaching and fungus development in the inner plant layer, a significant increasein plant litter decomposition was obtained at the !10 cm layer (Taylor & Parkinson,1988a, 1988b, 1988c, 1988d; Cochran, 1990). These findings support the hypothesisthat the active processes in the soil are mainly biological (Eijsakers & Zehnder, 1990;Beare et al., 1992).

Previous studies (Swift et al., 1979; Hunt et al., 1988; Slapokas, 1991; Taylor et al.,1991; Tripathi & Singhi, 1992) have discussed the importance of plant chemicalcomponents during decomposition. Cromack (1973) found that lignin impedes enzy-matic decomposition of carbohydrates and proteins, and that the nitrogen rate and thelignin:nitrogen ratio also influence the decomposition rate.

In A. sterilis litter, lignin rates were found to be at least 13% higher than in other plants(Melillo et al., 1982; Tripathi & Singhi, 1992). This high fraction can significantlyaffect decomposition rates after removal of the soluble fraction (Berg, 1984;Tripathi & Singhi, 1992). Our results are similar to those reported by Tripathi & Singhi(1992), who studied bambuk stems and lignin’s relation to plant material placed on thesoil surface. They found that after a pause in decomposition, fungus populations beginto grow and biological decomposition can begin (Swift et al., 1979; Slapokas, 1991).Fungi have a major influence on decomposition in plants with high lignin rates(Eijsackers & Zehnder, 1990). It is therefore important to examine the microbialpopulation character in order to determine the major factors that affect the de-composition process.

The increase in nitrogen from outside sources during the dry season could take placeby nitrogen fixing, as proposed by Hunt et al. (1988) and Taylor et al. (1991) or by dryaeolian dust deposition which is very common in arid systems (Offer et al., 1992).

The initial nitrogen values in the A. sterilis plants fluctuated between 6 to 9 mg g!1 drymatter. With low quantities of nitrogen, the nitrogen mineralized population proteins.When its rate was high, nitrogen did not limit the microbial population, and other factorssuch as climate and other nutrients became important (Swift et al., 1979; Taylor et al.,1991). The influence of nitrogen is mainly in huge plant lignin content, accounting forup to 25–30% of the decomposed material. The nitrogen rate in Avena sterilis is average(Cochran, 1990; Taylor et al., 1991; Tripathi & Singhi, 1992) and does not becomea limiting factor for the ground microbial populations. The lack of a relation betweennitrogen content and decomposition rate is in contradiction to Cochran (1990) and


Hunt et al.’s (1988) claim that an inverse relationship exists between nitrogen and thedecomposition rate. Thus, these data are different from those of Alexander (1977)and Berg & Staaf (1980), who found that nitrogen content affects the decomposi-tion rate during the first months of the study, but not afterwards. The lack of connectionbetween nitrogen content and the decomposition rate in Avena sterilis stems is probablydue to the high lignin content (50%) in the plant material. It is possible that abovea certain rate of lignin, the influence of other components decreases. Taylor et al. (1991)also obtained similar results: when the lignin content was above 23%, the componentsdid not affect the decomposition rate. This data and the original nitrogen rateexplain the lack of influence of the nitrogen in our case. There is an increase in the plantnitrogen rate during the year of study, probably because nitrogen from outside sourcesenters or because of nitrogen-fixation during the decomposition process (Swift et al.,1979; Hunt et al., 1988; Taylor et al., 1991). The entrance of nitrogen into the materialalso affects the lignin:nitrogen ratio, thus affecting the decomposition rate. Theprimary ratio in this study was more than 50:1. Melillo et al. (1982) found thata significant decrease in the decomposition rate occurs in plants in which thelignin:nitrogen ratio reaches these values. Taylor et al. (1991) found a decrease in thedecomposition rate even at a 30:1 ratio.

O’Brien (1978) claims that there is a connection between climate and the decomposi-tion rate and Comanor & Stafeldt (1978) found that temperature, rainfall rate and itsfrequency also influence the decomposition rate. The relationship between rainfall andits frequency and decomposition rate was also found in the savannas of India (Tripathi& Singhi, 1992). In our research no correlation was found between climate anddecomposition rate, probably due to the great influence of the lignin. This hypothesis issupported by the findings of Whitford et al. (1981) and Santos et al. (1984), who workedin four deserts in North America and found no correlation between rainfall anddecomposition rates. In their opinion, the reason for this is that the decomposerpopulations are adapted to desert conditions, which may explain our results as well.Studying plant material decomposition on the soil surface assumed that the number ofrain events, and not the amount of rain, affects the decomposition rate. However,this hypothesis was not verified in this research or in other studies. It is important topoint out that despite the similarity between our findings and those reported earlier, theclimate effects were isolated only in this study.

It is surprising how much similarity and dissimilarity can be found in a relativelysimple process such as the decomposition of organic matter. In such a process, whichrequires an organic to inorganic cycle for primary production to continue and isa function of chemical and physical properties of the litter, biotic and abiotic environ-mental conditions still remained unclear. However, the climatic patterns along thetopoclimatic gradient are obviously important in shaping the structure and function ofsoil communities similar to the North American hot deserts.

This research was supported by a grant from the Ministry of Science and from the KFA-BEO—Forschungszentrum Juelic GmbH/Projecttraeger fuer Biologie, Energie, und Oekologie.

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Decomposition of Avena sterilis litter under arid conditions