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The content and contributions of deposited aeolian organic matter in a dry land ecosystem of the Negev Desert, Israel
Atmospheric Environment 35 (2001) 769}776
The content and contributions of deposited aeolian organic matter in a dry land ecosystem of the Negev Desert, Israel E. Zaady *, Z.Y. O!er, M. Shachak Desertixcation and Restoration Ecology Research Center, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Center for Environmental Physics, Desert Meteorology Unit, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Mitrani Department for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Received 12 November 1999; accepted 10 May 2000
Abstract The northern Negev desert in Israel has a mosaic of two types of plant community patches. One is dominated by vascular plants (shrub patches) and the other by a nonvascular crust community (crust patches) consisting of cyanobacteria, bacteria, algae, mosses and lichens. The crust patches are sources of soil material and sediment } laden runo! water while the shrub patches are the sinks and function as &islands of fertility' in the desert environment. Accumulation of resources is often a limiting factor in this ecosystem. The aim of this study was to investigate the contribution of high nutrient organic residues that are readily decomposable, to the aeolian deposition on the crust patches. During the "ve years of the study, three dominant groups were found in the organic matter: plant material, insect and snail residue (feces). The average accumulation of the aeolian deposition of the organic matter showed signi"cant spatial and temporal di!erences. Similar quantities were found on the north and the south-facing slopes of the watershed with a minimum in the wadi. A signi"cant di!erence in average accumulated organic matter for the "ve years was found. The maximum was for the third year (1995) and the minimum for the "fth year (1997). This may be due to the variations in the annual rainfall amount. The results showed that the origin of most of the organic matter content is from native sources. The highest and most signi"cant amount was registered during the spring season, which is the #owering season in the northern Negev. This may explain the high quantity of plant material obtained and the great amounts of insect and snail (feces) residue that were also found during this season. Since the crust patches serve as a source of water and nutrients in this ecosystem, the organic residues that are of high nutrient quality and readily decomposable, contribute to the productivity of the shrub patches and thus of the overall ecosystem. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aeolian deposition; Organic matter; Arid landscape
1. Introduction In desert regions, dust is dispersed from di!erent sources. These include alluvial deposits on dry #ood plains,
* Corresponding author. Tel.: #972-7-6596784; fax: #9727-6596772. E-mail address: [email protected] (E. Zaady).
wadis, playas, etc. In addition to wind stress, dust production is in#uenced by vegetation, soil structure, moisture content of the soil, texture, mineral content and surface roughness (Goossens and O!er, 1988; Pewe, 1981). One of the most problematic results of the topsoil erosion is the repeated blowing away of the "ne particle fraction, as this upper layer contains most of the nutrients (O!er et al., 1993). The Negev desert in Israel (Fig. 1) has been under thousands of years of grazing. It consists of a mosaic of
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 6 3 - 6
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Fig. 1. Location of Sayeret Shaked Park on the 200-mm isohyet in the northern Negev desert of Israel (modi"cation from Stern et al., 1986).
two patches of plant communities (Noy-Meir, 1985; Shachak et al., 1998), shrubs interspersed in a matrix of plants of lower structure (Noy-Meir, 1985). This patchiness may indicate that the limited amount of water cannot support a high cover of shrubs (Tongway and Ludwig, 1994), or it may be a consequence of overgrazing, which leads to converting semiarid grasslands into shrublands (Schlesinger et al., 1990). The two patch types in the landscape are (Fig. 2): shrub patches dominated by vascular plants and nonvascular crust patch communities consisting of cyanobacteria, bacteria, algae, mosses and lichens (Eldridge and Green, 1994; Johansen, 1993; Shachak et al., 1998; West, 1990) growing on soil crusts. Studies in deserts of Arizona, Australia, Patagonia, and the Negev have shown that crust patches are sources for soil materials and runo! water, while shrub patches are sinks (Abrahams et al., 1994; Eldridge and Green, 1994; Rostango, 1989; Stockton and Gillete, 1990; Shachak et al., 1998; Yair and Shachak, 1987). Shrub patches are the principal loci of productivity and diversity. This is mainly because of the accumulation of water and nutrients, which support higher levels of plant growth and biomass, and enhance species diversity (Allen, 1991; Boeken and Shachak, 1994; Garner and Steinberger,
Fig. 2. Conceptual diagram of the Negev desert landscape showing shrub (macrophytic) patches and soil crust (microphytic) patches dominated by algae, cyanobacteria, lichens, mosses and soil crust.
1989; Noy-Meir, 1985; Schlesinger et al., 1990; Weinstein, 1975; West, 1989; Zaady et al., 1996a). Crust patches also play important roles in the ecosystem. They tend to reduce soil erosion by wind and water (Eldridge and Kinnell, 1997; Williams et al., 1995; McKenna-Neuman et al., 1996) and often contribute nitrogen through N "xation by cyanobacteria (Belnap, 1996; Evans and Ehleringer, 1993; Harper and Marble, 1988). In some cases, crusts enhance water in"ltration (Brotherson and
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Rushforth, 1983; Green et al., 1990) while in other cases, such as in our study, the crusts decrease in"ltration (Eldridge et al., 2000; Shachak et al., 1998; Yair, 1990). This redistribution of resources results in the formation of &islands of fertility' (Garner and Steinberger, 1989; Schlesinger et al., 1990; Tongway and Ludwig, 1994). The crust has a tightly structured surface (Fletcher and Martin, 1948), primarily due to the binding of soil particles by polysaccharides excreted by cyanobacteria (Schulten, 1985). In contrast, the soil of shrub patches lacks a well-developed crust, and its surface is covered with loose soil particles (Shachak et al., 1998). The system is maintained by feedback mechanisms, which allow for the #ow of resources between the various patches (Zaady and Shachak, 1994). Crusts also play major roles in seedling emergence and establishment (Harper and Marble, 1988; Eldridge et al., 1995; Zaady et al., 1997). O!er et al. (1998) have investigated the airborne particle accumulation and composition of aeolian deposition of dry sediments in the northern Negev desert. Shachak and Lovett (1998) have studied atmospheric deposition to desert ecosystems and its implications for management. They showed that "ne particles, consisting mostly of mineral dust, are deposited at similar rates in shrub and crust patches (Shachak and Lovett, 1998). Nevertheless, Shachak and Lovett (1998) did not analyze the origin of the content of the organic matter contributed by atmospheric deposition to the crust patches and the sediment- laden runo! water to the shrub patches. The readily decomposable organic matter deposited on the crust patches, which #ows to the shrub patches, contributes to the function of the ecosystem. In this study, we hypothesize that the content of the organic matter in the aeolian deposition may help control shrub patch fertility. Therefore, the aim of this study was to investigate the content of the organic matter input by aeolian deposition and its origin, in the northern Negev desert. For 5 years, we analyzed organic matter content in the aeolian deposition on the soil crust patches. In addition, we determined whether deposited organic matter might come from native sources such as plant and animal residues.
2. Study area and methods The study was carried out in Sayeret Shaked Park, long-term ecological research (LTER) station, near Beer-Sheva in the northern Negev of Israel (31317 N, 34337 E) (Fig. 1) (Stern et al., 1986), elevation 75}150 m above mean sea level. The landscape consists of gentle hills. The area has been closed to livestock grazing since 1987, "ve years prior to this study. Rainfall, which only occurs in winter between November and March, has a long-term annual average of 200 mm. The average air humidity is 40%. Average daily winter temperature is
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123C, and average daily maximum summer temperature is 253C (Zaady et al., 1996a). The 200 mm isohyet forms the transition zone between the arid and semiarid desert in Israel (Bruins, 1990) (Fig. 1). Distinct shrub patches within crust patches characterize spatial heterogeneity in the landscape. Shrub patches occur at a density of
12,000 patches ha\ and cover 20% of the area (Shachak et al., 1998). The soil surface is covered with soil crusts, consisting of bacteria, cyanobacteria, algae, mosses and lichens (Eldridge et al., 2000; Zaady et al., 1996a, 1997), and with scattered patches of shrubs; Noeae mucronata and Atractylis serratuloides (30}50 cm high) (Feinbrun-Dothan and Danin, 1991). The soil is loessial, about 1 m thick with 14% clay, 27% silt and 59% sand (USA classi"cation: loess soil with sandy loam texture } Calcixerollic, Xerochrepts). Salt content of the 0}25 cm soil layer is low, with electrical conductivity of 0.04 S m\. Five plots (10 m) were established along a cross section in the watershed (about 1 km) (O!er et al., 1998). The cross section was oriented from northeast to southwest, perpendicular to the wadi and to the predominant wind directions: which are northwest to southeast during the day and opposite at night. Two plots were placed on the south-facing slope (with an angle of ca. 15%), two on the north-facing slope (with an angle of ca. 7%) and one near the wadi (on the north side, on the wadi shoulder, near the lower part of the watershed, in order to avoid being washed away by #ooding in the wadi). At each plot, we set up six aeolian deposition collector pans (47;31;10 cm) containing a layer of spherical glass marbles (17 mm diameter) (Goossens and O!er, 1994; O!er et al., 1998). The six collectors were set up perpendicular to the slope direction, at 1 m distance between them and protruding 5 cm above the soil surface level. The rim of the collector was rounded and its edge was bent to the ground (X-shape), in order to prevent insects and ants from getting into the collectors. Sediment samples were removed from the pans each month by brushing the marbles and the pans. This method was chosen in order to prevent organic matter particles from disintegrating or dissolving in the water, and evaporating during the dry period. This is of particular importance with organic matter containing compounds, such as ammonium, present in the snail feces (Zaady et al., 1996b). We are aware of the error factor, which is about 5}8%, as described by Goossens and O!er (1994). The possibility of rain splash contributions into the traps from the surrounding soil was considered. However, since we trapped particles moving by saltation, higher collectors would have prevented that. The layer of spherical glass marbles and the 10 cm depth of the collector minimize the possibility of rain splash losses from within the collectors (O!er et al., 1998). The measurements of the components of the aeolian deposition and accumulation, at the study site of the
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Sayeret Shaked Ecological Park, were carried out for "ve years from August 1992 to July 1997. The oven-dried samples (403C for 7 days) were strained through sieves ((100 lm) to separate the major mineral dust and organic matter particles. Thus, particles (over 100 lm), which were undoubtedly identi"ed as organic matter, were separated from the soil aggregates by microscopic separation (;40). The organic matter was divided into di!erent source groups, until the particle size was too small for identi"cation (less than 100 lm). These small particles and other aggregates (mainly small crust aggregates and, in one case, lizard feces and lizard tail remains of Medalina guttalata were not included in the results. 2.1. Statistical analysis Data were analyzed using analysis of variance with the SuperANOVA statistical package. One-way ANOVA, with Duncan Multiple Range Tests and Sche!e F-tests (Sokal and Rohlf, 1995) were used to test for di!erences in the experimental years, seasons, months and the location on the slope and total aeolian deposition of dry sediments, the total identi"ed organic matter and the three main source groups separately. Experimental year, seasons, months and the location on the slope were independent variables. Dependent variables were the total identi"ed organic matter and each of the three source groups (Sokal and Rohlf, 1995).
3. Results The measurements of the components of the aeolian deposition and accumulation of the organic matter, at the study site of the Sayeret Shaked Park, were carried
out for "ve years from August 1992 to July 1997. The amount of rainfall during the wet winter season in the experimental years varied throughout the years; 1992}1993 } 157 mm, 1993}1994 } 97.5 mm, 1994}1995} 285 mm, 1995}1996 } 108 mm and 1996}1997 } 200 mm. The aeolian deposition-accumulation of atmospheric particles was investigated in the Negev highlands (Goossens and O!er, 1995). For their studies, the amount of deposition varied from 145 to 290 g m\ yr\, which is of the same order as the accumulation reported in previous studies at other sites (Goossens and O!er, 1990; O!er et al., 1992; Yaalon and Ganor, 1979). The average aeolian deposition accumulation for the "ve years of the present research was 161 g m\. The average accumulation of the aeolian deposition of the organic matter showed signi"cant spatial and temporal di!erences. Along the cross section of the watershed, di!erent amounts were measured. In the wadi and on the facing south slope about 30% more aeolian deposition was registered than on the facing north slope. There were no di!erences due to the location along the watershed cross section related to the average accumulated organic matter in the aeolian deposition (Fig. 3). Three dominant source groups were found in the organic matter components: plant material, insect and snail residue. The plant category contained dead parts and seeds of about 50 di!erent species. Dominant annuals are Stipa capensis, Bromus fasciculatus, Rostraria cristata, Avena barbata, Trifolium tomentosum, and Senecio glaucus, dominant shrub species are Atractylis serratuloides and Noeae mucronata. Same quantities were found on the north and the south-facing slopes of the watershed and the minimum in the wadi (Fig. 3). The second group that dominated the organic matter was insect residue. The insect category contains ants (Messor sp., Cataglyphis sp.), #ies (Anthaphorinae sp., Musca sp. and Lyperosia sp.) and
Fig. 3. The average aeolian deposition, organic matter and the three main source groups found: plant, insect and snail residue accumulated, by their location in the watershed. Values with di!erent letters are signi"cant (p(0.01).
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beetles (Potosia sp., Adesmia sp. and Reduvins sp.). A high and signi"cant amount of insect residue was registered in the wadi compared to the facing north and south slopes (Fig. 3). The third group that was predominant was the organic matter content of the snail residue (mainly feces). The snail category includes species such as Sphincterochila zonata and Trochoidea seetzenii. No signi"cant di!erences were found related to the location along the cross section of the watershed (Fig. 3). Among the "ve study years, the highest values were registered during the third year (1995) (358 g m\). High amount of aeolian deposition can be related to the rainfall e!ects throughout atmospheric washing. Related to the seasons, the maximum total aeolian deposition was measured during the winter season (62.925 g m\) and the minimum in the summer (9.537 g m\). Low amounts were found during the spring and autumn seasons. Signi"cant di!erences between the "ve study years were found related to the average accumulated organic matter in the aeolian deposition. The maximum was in winter 1994}1995 year and the minimum in the 1996}1997 year (Fig. 4). Between the seasons, the amount accumulated was highest in the spring and lowest in the autumn (Fig. 5). Concerning plant category, the most signi"cant amount accumulated during the third study year (1995), with more than eight times, compared to the other years (Fig. 4). The highest and most signi"cant amount was registered during the spring season and the minimum in the winter season. No signi"cant di!erences were found between the summer and autumn (Fig. 5). The maximum quantity of insect residue was measured during the third study year and the minimum during the "fth year (1997). Signi"cant decrease in the amount was found during the fourth and the "rst year (1993) (Fig. 4). As in the previous category, the highest and most signi"cant amount was measured during the spring season. Decreases were found from winter to the summer and autumn (Fig. 5).
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Among the study years the maximum and most signi"cant amounts of snail residues were measured during the fourth year followed by the "rst year. The minimum quantities were found in the second, third and the "fth years (1997) (Fig. 4). As with the previous categories, the highest and most signi"cant amount was measured during the spring season. No signi"cant di!erences were measured between the winter and the summer seasons while the minimum was registered in the autumn (Fig. 5).
4. Discussion Our results show that the origin of most of the organic matter is from native sources. It was found that the biotic part of the total aeolian deposition belongs to three main groups of organisms: plants, insects and snails. Boeken and Shachak (1994), Feinbrun-Dothan and Danin (1991), Shachak et al. (1998) and Yair and Shachak (1987) studied the characteristics of plants and animals in the northern Negev desert ecosystems. By comparison of their studies with our results, we conclude that the organic elements deposited in Sayeret Shaked Park originated from biological sources in the northern Negev ecosystems. As an outcome of thousand years of grazing, the shrub patches in this area are colonized by two dominant thorny shrub species: Atractylis serratuloides and Noeae mucronata, and common annual species Stipa capensis, Avena barbata and Bromus fasciculatus (Boeken and Shachak, 1994). Dead fragments of these plants and their seeds were found to constitute most of the plant material accumulated as part of the organic matter in the aeolian deposition. Because, grazing was excluded from the studied watershed, soil moisture became the main factor a!ecting plant growth. As a result, when rainfall increased the relative proportion of the plant material in the organic matter increased (Fig. 2).
Fig. 4. The average accumulation of organic matter and the three main source groups found: plant, insect and snail residue for each experimental year. Values with di!erent letters are signi"cant (p(0.01).
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Fig. 5. The average aeolian deposition, organic matter and the three main source groups found: plant, insect and snail residue accumulated by seasons. Values with di!erent letters are signi"cant (p(0.01).
Concerning the spatial distribution, di!erences were found between the slopes. The north-facing slope is higher in soil moisture in that it receives less solar radiation, although not signi"cant statistically. It showed about a 20% higher amount of plant material than for the south-facing slope. Most of the plants found in the wadi are shrubs that grow in the channel and on the wadi shoulders, where our plot was situated. These shrubs contribute less plant material to the accumulated biomass than the annuals. The insect residues that were found, contained ants, #ies and beetles. Most insects prefer the wadi habitat, which is more humid, compared to the slopes. Regarding the snails, they exhibited no preferences for locations across the watershed, therefore no residue di!erences were obtained among the two slopes and the wadi. With respect to the temporal scale, the #owering season in the northern Negev desert is during the spring. This may explain the high quantity of plant material (76%) accumulated in the spring compared to the other seasons. Most of the insect residues also appeared in the spring season (63%). In addition, the highest accumulated snail feces residues were also measured in the spring season (38%). The snails feed on vegetation and accumulated feces. Plants, as producers, are the base of the ecological pyramid. They serve as food for herbivores such as snails and insects. The amount of vegetation determines the amount of herbivores and predators that the system can support. This may explain, the high amount of insects and snails found in the spring season. Shachak and Lovett (1998) studied the possible contribution of the total organic matter C and N to the system by aeolian deposition. They di!erentiated between the N of "ne particle (less than &0.5 mm diameter) input, 0.6}1.4 mg m\ and coarse particles (greater than &0.5 mm diameter), 0.3}0.4 mg m\. They showed the
same distribution for the C element. For the "ne particles C: 8}88 mg m\ and for the coarse particles (which are primarily composed of organic material from plant detritus) C: 5}10 mg m\ (Shachak and Lovett, 1998). Zaady et al. (1996b) found that snail feces contribute high amounts of nitrogen to the systems. They estimated this contribution to be about 38 g m\ yr\ (49%) of the total N budget in this ecosystem. The main component of the insect body that remains as residue is the chitin, which contains high amounts of N (Paul and Clark, 1996). The three components found in the present study predominated in the organic matter deposited as part of the aeolian deposition, on the soil crust patches in this ecosystem. The crust patches serve as sources of water and nutrients to the shrub patches (Abrahams et al., 1994; Shachak et al., 1998), and help in the formation of &islands of fertility' (Garner and Steinberger, 1989; Schlesinger et al., 1990; Tongway and Ludwig, 1994). Our results emphasize the important role of these deposited organic materials. Since crust patches provide resources in this ecosystem, the organic residues are of high nutrient quality and readily decomposable; they contribute to control the productivity of the shrub patches and thus of the overall ecosystem. Our aeolian deposition collectors indicate the deposition of dry material on the soil crusts. Under natural conditions, after a rainfall event, a relatively large amount of dry deposition is transported by sediment} laden runo! water to short distances in the watershed and out of the system, by atmospheric dynamics. Therefore, because of their importance, it is necessary to retain these resources in the system, in order to increase diversity and productivity of arid landscapes. We suggest that future research should concentrate on "nding practical solutions for conserving deposited aeolian material on the soil crust patches.
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Acknowledgements We thank Sonia Rozin and Stefanie Herrmann for the technical assistance and Sol Brand for reviewing the manuscript. This paper is a publication of the Deserti"cation and Restoration Ecology Research Center, Ben-Gurion University of the Negev.
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Yaalon, D., Ganor, E., 1979. East Mediterranean trajectories of dust-carrying storms from the Sahara and Sinai. In: Morales, C. (Ed.), Saharan Dust: Mobilization, Transport, Deposition. Wiley, New York, pp. 187}193. Yair, A., 1990. Runo! generation in a sandy area } the Nizzana Sands, western Negev, Israel. Earth Surface Processes and Landforms 15, 597}609. Yair, A., Shachak, M., 1987. Studies in watershed ecology of an arid area. In: Berkofsky, L., Wurtele, M.G. (Eds.), Progress in Desert Research. Rowman and Little"eld, Totowa, USA, pp. 145}193. Zaady, E., Shachak, M., 1994. Crustsoil crust and ecosystem leakage in the Negev Desert. American Journal of Botany 81, 109. Zaady, E., Gro!man, P., Shachak, M., 1996a. Litter as a regulator of nitrogen and carbon dynamics in shrub patches in Negev Desert soils. Soil Biology and Biochemistry 28, 39}46. Zaady, E., Gro!man, P., Shachak, M., 1996b. Release and consumption of snail feces in Negev Desert soils. Biology and Fertility of Soils 23, 399}404. Zaady, E., Gutterman, Y., Boeken, B., 1997. The germination e!ects of cyanobacterial soil crust on mucilaginous seeds of three desert plants: Plantago coronopus, Reboudia pinnata and Carrichtera annua. Plant and Soil 190, 247}252.
The content and contributions of deposited aeolian organic matter in a dry land ecosystem of the Negev Desert, Israel E. Zaady *, Z.Y. O!er, M. Shachak Desertixcation and Restoration Ecology Research Center, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Center for Environmental Physics, Desert Meteorology Unit, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Mitrani Department for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, 84990 Sede Boker Campus, Israel Received 12 November 1999; accepted 10 May 2000
Abstract The northern Negev desert in Israel has a mosaic of two types of plant community patches. One is dominated by vascular plants (shrub patches) and the other by a nonvascular crust community (crust patches) consisting of cyanobacteria, bacteria, algae, mosses and lichens. The crust patches are sources of soil material and sediment } laden runo! water while the shrub patches are the sinks and function as &islands of fertility' in the desert environment. Accumulation of resources is often a limiting factor in this ecosystem. The aim of this study was to investigate the contribution of high nutrient organic residues that are readily decomposable, to the aeolian deposition on the crust patches. During the "ve years of the study, three dominant groups were found in the organic matter: plant material, insect and snail residue (feces). The average accumulation of the aeolian deposition of the organic matter showed signi"cant spatial and temporal di!erences. Similar quantities were found on the north and the south-facing slopes of the watershed with a minimum in the wadi. A signi"cant di!erence in average accumulated organic matter for the "ve years was found. The maximum was for the third year (1995) and the minimum for the "fth year (1997). This may be due to the variations in the annual rainfall amount. The results showed that the origin of most of the organic matter content is from native sources. The highest and most signi"cant amount was registered during the spring season, which is the #owering season in the northern Negev. This may explain the high quantity of plant material obtained and the great amounts of insect and snail (feces) residue that were also found during this season. Since the crust patches serve as a source of water and nutrients in this ecosystem, the organic residues that are of high nutrient quality and readily decomposable, contribute to the productivity of the shrub patches and thus of the overall ecosystem. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Aeolian deposition; Organic matter; Arid landscape
1. Introduction In desert regions, dust is dispersed from di!erent sources. These include alluvial deposits on dry #ood plains,
* Corresponding author. Tel.: #972-7-6596784; fax: #9727-6596772. E-mail address: [email protected] (E. Zaady).
wadis, playas, etc. In addition to wind stress, dust production is in#uenced by vegetation, soil structure, moisture content of the soil, texture, mineral content and surface roughness (Goossens and O!er, 1988; Pewe, 1981). One of the most problematic results of the topsoil erosion is the repeated blowing away of the "ne particle fraction, as this upper layer contains most of the nutrients (O!er et al., 1993). The Negev desert in Israel (Fig. 1) has been under thousands of years of grazing. It consists of a mosaic of
1352-2310/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 0 0 ) 0 0 2 6 3 - 6
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Fig. 1. Location of Sayeret Shaked Park on the 200-mm isohyet in the northern Negev desert of Israel (modi"cation from Stern et al., 1986).
two patches of plant communities (Noy-Meir, 1985; Shachak et al., 1998), shrubs interspersed in a matrix of plants of lower structure (Noy-Meir, 1985). This patchiness may indicate that the limited amount of water cannot support a high cover of shrubs (Tongway and Ludwig, 1994), or it may be a consequence of overgrazing, which leads to converting semiarid grasslands into shrublands (Schlesinger et al., 1990). The two patch types in the landscape are (Fig. 2): shrub patches dominated by vascular plants and nonvascular crust patch communities consisting of cyanobacteria, bacteria, algae, mosses and lichens (Eldridge and Green, 1994; Johansen, 1993; Shachak et al., 1998; West, 1990) growing on soil crusts. Studies in deserts of Arizona, Australia, Patagonia, and the Negev have shown that crust patches are sources for soil materials and runo! water, while shrub patches are sinks (Abrahams et al., 1994; Eldridge and Green, 1994; Rostango, 1989; Stockton and Gillete, 1990; Shachak et al., 1998; Yair and Shachak, 1987). Shrub patches are the principal loci of productivity and diversity. This is mainly because of the accumulation of water and nutrients, which support higher levels of plant growth and biomass, and enhance species diversity (Allen, 1991; Boeken and Shachak, 1994; Garner and Steinberger,
Fig. 2. Conceptual diagram of the Negev desert landscape showing shrub (macrophytic) patches and soil crust (microphytic) patches dominated by algae, cyanobacteria, lichens, mosses and soil crust.
1989; Noy-Meir, 1985; Schlesinger et al., 1990; Weinstein, 1975; West, 1989; Zaady et al., 1996a). Crust patches also play important roles in the ecosystem. They tend to reduce soil erosion by wind and water (Eldridge and Kinnell, 1997; Williams et al., 1995; McKenna-Neuman et al., 1996) and often contribute nitrogen through N "xation by cyanobacteria (Belnap, 1996; Evans and Ehleringer, 1993; Harper and Marble, 1988). In some cases, crusts enhance water in"ltration (Brotherson and
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Rushforth, 1983; Green et al., 1990) while in other cases, such as in our study, the crusts decrease in"ltration (Eldridge et al., 2000; Shachak et al., 1998; Yair, 1990). This redistribution of resources results in the formation of &islands of fertility' (Garner and Steinberger, 1989; Schlesinger et al., 1990; Tongway and Ludwig, 1994). The crust has a tightly structured surface (Fletcher and Martin, 1948), primarily due to the binding of soil particles by polysaccharides excreted by cyanobacteria (Schulten, 1985). In contrast, the soil of shrub patches lacks a well-developed crust, and its surface is covered with loose soil particles (Shachak et al., 1998). The system is maintained by feedback mechanisms, which allow for the #ow of resources between the various patches (Zaady and Shachak, 1994). Crusts also play major roles in seedling emergence and establishment (Harper and Marble, 1988; Eldridge et al., 1995; Zaady et al., 1997). O!er et al. (1998) have investigated the airborne particle accumulation and composition of aeolian deposition of dry sediments in the northern Negev desert. Shachak and Lovett (1998) have studied atmospheric deposition to desert ecosystems and its implications for management. They showed that "ne particles, consisting mostly of mineral dust, are deposited at similar rates in shrub and crust patches (Shachak and Lovett, 1998). Nevertheless, Shachak and Lovett (1998) did not analyze the origin of the content of the organic matter contributed by atmospheric deposition to the crust patches and the sediment- laden runo! water to the shrub patches. The readily decomposable organic matter deposited on the crust patches, which #ows to the shrub patches, contributes to the function of the ecosystem. In this study, we hypothesize that the content of the organic matter in the aeolian deposition may help control shrub patch fertility. Therefore, the aim of this study was to investigate the content of the organic matter input by aeolian deposition and its origin, in the northern Negev desert. For 5 years, we analyzed organic matter content in the aeolian deposition on the soil crust patches. In addition, we determined whether deposited organic matter might come from native sources such as plant and animal residues.
2. Study area and methods The study was carried out in Sayeret Shaked Park, long-term ecological research (LTER) station, near Beer-Sheva in the northern Negev of Israel (31317 N, 34337 E) (Fig. 1) (Stern et al., 1986), elevation 75}150 m above mean sea level. The landscape consists of gentle hills. The area has been closed to livestock grazing since 1987, "ve years prior to this study. Rainfall, which only occurs in winter between November and March, has a long-term annual average of 200 mm. The average air humidity is 40%. Average daily winter temperature is
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123C, and average daily maximum summer temperature is 253C (Zaady et al., 1996a). The 200 mm isohyet forms the transition zone between the arid and semiarid desert in Israel (Bruins, 1990) (Fig. 1). Distinct shrub patches within crust patches characterize spatial heterogeneity in the landscape. Shrub patches occur at a density of
12,000 patches ha\ and cover 20% of the area (Shachak et al., 1998). The soil surface is covered with soil crusts, consisting of bacteria, cyanobacteria, algae, mosses and lichens (Eldridge et al., 2000; Zaady et al., 1996a, 1997), and with scattered patches of shrubs; Noeae mucronata and Atractylis serratuloides (30}50 cm high) (Feinbrun-Dothan and Danin, 1991). The soil is loessial, about 1 m thick with 14% clay, 27% silt and 59% sand (USA classi"cation: loess soil with sandy loam texture } Calcixerollic, Xerochrepts). Salt content of the 0}25 cm soil layer is low, with electrical conductivity of 0.04 S m\. Five plots (10 m) were established along a cross section in the watershed (about 1 km) (O!er et al., 1998). The cross section was oriented from northeast to southwest, perpendicular to the wadi and to the predominant wind directions: which are northwest to southeast during the day and opposite at night. Two plots were placed on the south-facing slope (with an angle of ca. 15%), two on the north-facing slope (with an angle of ca. 7%) and one near the wadi (on the north side, on the wadi shoulder, near the lower part of the watershed, in order to avoid being washed away by #ooding in the wadi). At each plot, we set up six aeolian deposition collector pans (47;31;10 cm) containing a layer of spherical glass marbles (17 mm diameter) (Goossens and O!er, 1994; O!er et al., 1998). The six collectors were set up perpendicular to the slope direction, at 1 m distance between them and protruding 5 cm above the soil surface level. The rim of the collector was rounded and its edge was bent to the ground (X-shape), in order to prevent insects and ants from getting into the collectors. Sediment samples were removed from the pans each month by brushing the marbles and the pans. This method was chosen in order to prevent organic matter particles from disintegrating or dissolving in the water, and evaporating during the dry period. This is of particular importance with organic matter containing compounds, such as ammonium, present in the snail feces (Zaady et al., 1996b). We are aware of the error factor, which is about 5}8%, as described by Goossens and O!er (1994). The possibility of rain splash contributions into the traps from the surrounding soil was considered. However, since we trapped particles moving by saltation, higher collectors would have prevented that. The layer of spherical glass marbles and the 10 cm depth of the collector minimize the possibility of rain splash losses from within the collectors (O!er et al., 1998). The measurements of the components of the aeolian deposition and accumulation, at the study site of the
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Sayeret Shaked Ecological Park, were carried out for "ve years from August 1992 to July 1997. The oven-dried samples (403C for 7 days) were strained through sieves ((100 lm) to separate the major mineral dust and organic matter particles. Thus, particles (over 100 lm), which were undoubtedly identi"ed as organic matter, were separated from the soil aggregates by microscopic separation (;40). The organic matter was divided into di!erent source groups, until the particle size was too small for identi"cation (less than 100 lm). These small particles and other aggregates (mainly small crust aggregates and, in one case, lizard feces and lizard tail remains of Medalina guttalata were not included in the results. 2.1. Statistical analysis Data were analyzed using analysis of variance with the SuperANOVA statistical package. One-way ANOVA, with Duncan Multiple Range Tests and Sche!e F-tests (Sokal and Rohlf, 1995) were used to test for di!erences in the experimental years, seasons, months and the location on the slope and total aeolian deposition of dry sediments, the total identi"ed organic matter and the three main source groups separately. Experimental year, seasons, months and the location on the slope were independent variables. Dependent variables were the total identi"ed organic matter and each of the three source groups (Sokal and Rohlf, 1995).
3. Results The measurements of the components of the aeolian deposition and accumulation of the organic matter, at the study site of the Sayeret Shaked Park, were carried
out for "ve years from August 1992 to July 1997. The amount of rainfall during the wet winter season in the experimental years varied throughout the years; 1992}1993 } 157 mm, 1993}1994 } 97.5 mm, 1994}1995} 285 mm, 1995}1996 } 108 mm and 1996}1997 } 200 mm. The aeolian deposition-accumulation of atmospheric particles was investigated in the Negev highlands (Goossens and O!er, 1995). For their studies, the amount of deposition varied from 145 to 290 g m\ yr\, which is of the same order as the accumulation reported in previous studies at other sites (Goossens and O!er, 1990; O!er et al., 1992; Yaalon and Ganor, 1979). The average aeolian deposition accumulation for the "ve years of the present research was 161 g m\. The average accumulation of the aeolian deposition of the organic matter showed signi"cant spatial and temporal di!erences. Along the cross section of the watershed, di!erent amounts were measured. In the wadi and on the facing south slope about 30% more aeolian deposition was registered than on the facing north slope. There were no di!erences due to the location along the watershed cross section related to the average accumulated organic matter in the aeolian deposition (Fig. 3). Three dominant source groups were found in the organic matter components: plant material, insect and snail residue. The plant category contained dead parts and seeds of about 50 di!erent species. Dominant annuals are Stipa capensis, Bromus fasciculatus, Rostraria cristata, Avena barbata, Trifolium tomentosum, and Senecio glaucus, dominant shrub species are Atractylis serratuloides and Noeae mucronata. Same quantities were found on the north and the south-facing slopes of the watershed and the minimum in the wadi (Fig. 3). The second group that dominated the organic matter was insect residue. The insect category contains ants (Messor sp., Cataglyphis sp.), #ies (Anthaphorinae sp., Musca sp. and Lyperosia sp.) and
Fig. 3. The average aeolian deposition, organic matter and the three main source groups found: plant, insect and snail residue accumulated, by their location in the watershed. Values with di!erent letters are signi"cant (p(0.01).
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beetles (Potosia sp., Adesmia sp. and Reduvins sp.). A high and signi"cant amount of insect residue was registered in the wadi compared to the facing north and south slopes (Fig. 3). The third group that was predominant was the organic matter content of the snail residue (mainly feces). The snail category includes species such as Sphincterochila zonata and Trochoidea seetzenii. No signi"cant di!erences were found related to the location along the cross section of the watershed (Fig. 3). Among the "ve study years, the highest values were registered during the third year (1995) (358 g m\). High amount of aeolian deposition can be related to the rainfall e!ects throughout atmospheric washing. Related to the seasons, the maximum total aeolian deposition was measured during the winter season (62.925 g m\) and the minimum in the summer (9.537 g m\). Low amounts were found during the spring and autumn seasons. Signi"cant di!erences between the "ve study years were found related to the average accumulated organic matter in the aeolian deposition. The maximum was in winter 1994}1995 year and the minimum in the 1996}1997 year (Fig. 4). Between the seasons, the amount accumulated was highest in the spring and lowest in the autumn (Fig. 5). Concerning plant category, the most signi"cant amount accumulated during the third study year (1995), with more than eight times, compared to the other years (Fig. 4). The highest and most signi"cant amount was registered during the spring season and the minimum in the winter season. No signi"cant di!erences were found between the summer and autumn (Fig. 5). The maximum quantity of insect residue was measured during the third study year and the minimum during the "fth year (1997). Signi"cant decrease in the amount was found during the fourth and the "rst year (1993) (Fig. 4). As in the previous category, the highest and most signi"cant amount was measured during the spring season. Decreases were found from winter to the summer and autumn (Fig. 5).
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Among the study years the maximum and most signi"cant amounts of snail residues were measured during the fourth year followed by the "rst year. The minimum quantities were found in the second, third and the "fth years (1997) (Fig. 4). As with the previous categories, the highest and most signi"cant amount was measured during the spring season. No signi"cant di!erences were measured between the winter and the summer seasons while the minimum was registered in the autumn (Fig. 5).
4. Discussion Our results show that the origin of most of the organic matter is from native sources. It was found that the biotic part of the total aeolian deposition belongs to three main groups of organisms: plants, insects and snails. Boeken and Shachak (1994), Feinbrun-Dothan and Danin (1991), Shachak et al. (1998) and Yair and Shachak (1987) studied the characteristics of plants and animals in the northern Negev desert ecosystems. By comparison of their studies with our results, we conclude that the organic elements deposited in Sayeret Shaked Park originated from biological sources in the northern Negev ecosystems. As an outcome of thousand years of grazing, the shrub patches in this area are colonized by two dominant thorny shrub species: Atractylis serratuloides and Noeae mucronata, and common annual species Stipa capensis, Avena barbata and Bromus fasciculatus (Boeken and Shachak, 1994). Dead fragments of these plants and their seeds were found to constitute most of the plant material accumulated as part of the organic matter in the aeolian deposition. Because, grazing was excluded from the studied watershed, soil moisture became the main factor a!ecting plant growth. As a result, when rainfall increased the relative proportion of the plant material in the organic matter increased (Fig. 2).
Fig. 4. The average accumulation of organic matter and the three main source groups found: plant, insect and snail residue for each experimental year. Values with di!erent letters are signi"cant (p(0.01).
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Fig. 5. The average aeolian deposition, organic matter and the three main source groups found: plant, insect and snail residue accumulated by seasons. Values with di!erent letters are signi"cant (p(0.01).
Concerning the spatial distribution, di!erences were found between the slopes. The north-facing slope is higher in soil moisture in that it receives less solar radiation, although not signi"cant statistically. It showed about a 20% higher amount of plant material than for the south-facing slope. Most of the plants found in the wadi are shrubs that grow in the channel and on the wadi shoulders, where our plot was situated. These shrubs contribute less plant material to the accumulated biomass than the annuals. The insect residues that were found, contained ants, #ies and beetles. Most insects prefer the wadi habitat, which is more humid, compared to the slopes. Regarding the snails, they exhibited no preferences for locations across the watershed, therefore no residue di!erences were obtained among the two slopes and the wadi. With respect to the temporal scale, the #owering season in the northern Negev desert is during the spring. This may explain the high quantity of plant material (76%) accumulated in the spring compared to the other seasons. Most of the insect residues also appeared in the spring season (63%). In addition, the highest accumulated snail feces residues were also measured in the spring season (38%). The snails feed on vegetation and accumulated feces. Plants, as producers, are the base of the ecological pyramid. They serve as food for herbivores such as snails and insects. The amount of vegetation determines the amount of herbivores and predators that the system can support. This may explain, the high amount of insects and snails found in the spring season. Shachak and Lovett (1998) studied the possible contribution of the total organic matter C and N to the system by aeolian deposition. They di!erentiated between the N of "ne particle (less than &0.5 mm diameter) input, 0.6}1.4 mg m\ and coarse particles (greater than &0.5 mm diameter), 0.3}0.4 mg m\. They showed the
same distribution for the C element. For the "ne particles C: 8}88 mg m\ and for the coarse particles (which are primarily composed of organic material from plant detritus) C: 5}10 mg m\ (Shachak and Lovett, 1998). Zaady et al. (1996b) found that snail feces contribute high amounts of nitrogen to the systems. They estimated this contribution to be about 38 g m\ yr\ (49%) of the total N budget in this ecosystem. The main component of the insect body that remains as residue is the chitin, which contains high amounts of N (Paul and Clark, 1996). The three components found in the present study predominated in the organic matter deposited as part of the aeolian deposition, on the soil crust patches in this ecosystem. The crust patches serve as sources of water and nutrients to the shrub patches (Abrahams et al., 1994; Shachak et al., 1998), and help in the formation of &islands of fertility' (Garner and Steinberger, 1989; Schlesinger et al., 1990; Tongway and Ludwig, 1994). Our results emphasize the important role of these deposited organic materials. Since crust patches provide resources in this ecosystem, the organic residues are of high nutrient quality and readily decomposable; they contribute to control the productivity of the shrub patches and thus of the overall ecosystem. Our aeolian deposition collectors indicate the deposition of dry material on the soil crusts. Under natural conditions, after a rainfall event, a relatively large amount of dry deposition is transported by sediment} laden runo! water to short distances in the watershed and out of the system, by atmospheric dynamics. Therefore, because of their importance, it is necessary to retain these resources in the system, in order to increase diversity and productivity of arid landscapes. We suggest that future research should concentrate on "nding practical solutions for conserving deposited aeolian material on the soil crust patches.
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Acknowledgements We thank Sonia Rozin and Stefanie Herrmann for the technical assistance and Sol Brand for reviewing the manuscript. This paper is a publication of the Deserti"cation and Restoration Ecology Research Center, Ben-Gurion University of the Negev.
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