Journal of Arid Environments (2001) 48: 267–280 doi:10.1006/jare.2000.0758, available online at http://www.idealibrary.com on
Temporal changes in nematode community structure in a desert ecosystem
W. Liang*- & Y. Steinberger*? *Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel ?Open Laboratory of Ecological Process of Trace Substance in Terrestrial Ecosystem, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110015, P. R. China ( Received 6 April 1999; accepted 24 October 2000; published electronically 21 May 2001) The effects of soil conditions on soil nematode community were studied. Soil samples from the upper soil layer (0–10 cm) were collected monthly (except for two samplings in February 1999), under the canopies of the two halophytic shrubs: Artemisia monosperma and Retama raetam. Significant differences in soil environmental variables (soil moisture, organic matter, electrical conductivity, Ca2>, K> and Na>) were observed between the canopies and the bare soil samples (p(0·01) during the study period. The soil water content under shrubs and in bare soil was (1·0% from April to December 1998 then increased to 4·9% in January 1999. During the winter season (December 1998 to March 1999), the mean numbers of total free-living nematodes were found to be lower under A. monosperma than under R. raetam. Various ecological indices were used to assess and compare the response of nematode populations to the soil conditions. From October 1998 to February 1999, the mean values of Simpson’s diversity index (SI), Shannon’s index (H ) and Sigma maturity index ( MI) were observed to be lower under A. monosperma than under R. raetam. The ratio of (fungivores#bacterivores) to plant parasites ((FF#BF)/PP), SI, H and MI were found to be sensitive to moisture stress in a sandy desert ecosystem. 2001 Academic Press Keywords: soil nematode; halophytes; desert salinity
community
structure;
ecological
indices;
Introduction Nematodes constitute a numerically important component of soil fauna in desert ecosystems (Freckman & Mankau, 1986; Steinberger et al., 1989), but an understanding of their responses to severe and rapidly changing environmental conditions is far from complete (Steinberger et al., 1989; Liang et al., 2000). Nematode populations play a significant role in the decomposition of soil organic matter, mineralization of plant nutrients and nutrient cycling (Ingham et al., 1985; Griffiths, 1994). Since nematodes are ubiquitous and occupy important positions in the detritus food web
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2001 Academic Press
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(Ingham et al., 1985), they have been used as indicators of overall ecological condition (Bongers, 1990; Freckman & Ettema, 1993; Neher et al., 1995; Wardle et al., 1995). The intersection of community analysis and ecosystem function could prove to be quite fruitful in the field of soil ecology (Coleman & Crossley, 1996). Arid soils frequently exhibit an accumulation of salts, which likely affect the distribution of soil nematode community (Powers et al., 1998). Nematodes were absent in soils with high inorganic ion accumulations, indicating that salinity may limit biotic activity in these soils (Freckman & Virginia, 1986). In the Negev Desert, the physical and chemical components of hilltop slope plain areas at the deeper layers, as well as the salinity of the underground water, determine the nature of the vegetation. Two halophyte shrubs: the white broom — Retama raetam and the one-seeded sagebrush — Artemisa monosperma, are typical of the sandy areas in northern Negev Desert (Evenari et al., 1982). The total salt and chloride ion content of the soils is an important factor differentiating between the habitats in the study area, where high salinity may further decrease the plant’s capacity to keep itself supported with water (Evenari et al., 1982). Steinberger & Loboda (1991) found that root biomass of Zygophyllum dumosum had an important effect on root-nematode dynamics in the northern Negev loess plains located at the Avdat Research Farm. Analysis of the soil nematode community structure under the canopy of A. monosperma and R. raetam can help in increasing our understanding of the responses of nematode communities to the stressed soil habitats. The objectives of this study were to monitor the seasonal dynamics of nematode community under the canopy of the two shrubs (A. monosperma and R. raetam), and to determine the relationship between soil conditions and free-living nematode community in a sandy desert ecosystem. Materials and methods The fieldwork in this study was conducted in the northern Negev at Bira-Sluge (31304 N; 34342 E), approximately 24 km south of Beer Sheva. This area has a temperate desert climate, i.e. mild rainy winters (10–123C range in January) and hot dry summers (26–283C in August). Radiation may reach 3·14;104 kJ m\2 d\1. The average multiannual rainfall is 100 mm, and the potential annual evaporation rate is ca. 1700 mm. The soil is sand dunes. The topography of the area is relatively even, and the predominant plants are two bushes, the white broom, R. raetam, and the one-seeded sagebrush, A. monosperma (Evenari et al., 1982). Soil samples from the upper soil layer (0–10 cm) were collected monthly during April 1998 and March 1999 (except for an additional two samplings on 18 and 22 February 1999, the latter taken after 4 mm of rainfall event), under the canopy of four individual plants (n"4) of each of the two plant species: A. monosperma and R. raetam. The control samples were taken from an open space between the plants. After sampling, the soils were placed in an insulation container transported to the laboratory, and stored at the existing moisture and 153C to minimize changes in nematode populations (Barker et al., 1986). Subsamples were taken from each such sample for estimation of nematode populations and different soil parameters. Soil moisture was determined gravimetrically by drying samples at 1053C for 48 h, and expressed as a percentage of dry weight. Soil organic matter (AFDM"ash-free dry mass) content was determined by burning dried soil in a muffle furnace at 4903C for 8 h. Soil salinity was determined in soil extracts and expressed as electrical conductivity (lS g\1). Soluble cations (Ca2>, K>, Na>) were determined by an atomic absorption spectrometer (Rhoades, 1982). Nematodes were extracted from 100-g composite samples using the Baermann funnel procedure (Cairns, 1960). All the data are expressed on dry mass basis. The organisms recovered were counted and preserved in formalin (Steinberger & Sarig, 1993). A number of randomly selected organism of each sample was identified to genus level, if
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possible, using an inverted compound microscope. The classification of trophic groups was assigned to: (1) bacterivores; (2) fungivores; (3) plant-parasites; and (4) omnivorespredators (Steinberger & Sarig, 1993; Yeates et al., 1993b; Liang et al., 2000). The nematode community was analysed by the following approaches: (1) absolute abundance of individuals 100 g\1 dry soil; (2) trophic structure; (3) ratio of (fungivores#bacterivores) to plant parasites [(FF#BF)/PP], which can present substantial changes in the trophic structure of the community (Wasilewska, 1994); (4) fungivore/bacterivore ratio (F/B), which indicates the organic matter decomposition pathway in detrital food webs, lower ratios being associated with higher rates of decomposition and nutrient turnover (Twinn, 1974); (5) modified F/B, ratio of fungivores to (fungivores#bacterivores) (Yeates et al., 1993a; Neher & Campbell, 1994); (6) Simpson diversity index (SI), which gives more weight to common genera, with SI"1/ Pi2, in which Pi is the proportion of each taxon in the total population (Freckman & Ettema, 1993); (7) Shannon index (H ), a species diversity measure, which gives more weight to rare species, H "! Pi(ln Pi), where Pi is the proportion of each trophic group in the total population (Pielou, 1975); (8) Modified maturity index ( MI), the nematode maturity index (MI) of Bongers (1990) is modified to include plant feeding nematodes and thus better reflects ecosystem development (Yeates, 1994). MI incorporates ecological characteristics of families based on a colonizer-to-persister scale of 1–5, lower MI values indicating more disturbed environments. All the data were subjected to statistical analysis of variance (ANOVA) using SAS software package (Statistical Analysis Systems Institute Inc., 1989) to evaluate sampling time and treatment effects. Differences between means were tested for significance by Duncan Multiple Range Test (DMRT) at the 5% level (p(0·05). Results Soil conditions Significant differences in soil environmental variables (soil moisture, organic matter, electrical conductivity, Ca2>, K> and Na>) were observed between treatments (p(0·01, n"156) (Table 1) during the study period. The contents of soil moisture under two shrubs and control soil during April and December 1998 were no more than 0%, then increased sharply in January 1999 and reached maximum values on 22 February 1999 (Fig. 1a). The values of soil electrical conductivity across three samplings (24 December 1998, 24 January and 18 February 1999) were less under A. monosperma than under R. raetam. The maximum values of electrical conductivity under A. monosperma and R. raetam were found on 18 and 22 February 1999, respectively (Fig. 2a). Soil moisture exhibited a negative correlation with the contents of organic matter and cations (Ca2>, K>, Na>) (p(0·05, n"156). A positive correlation was found between soil moisture and electrical conductivity (p(0·01, n"156). Nematode community structure Sixteen nematode families and 20 genera were found in the nematode suspensions (Table 2). Cephalobus, Rhabditidae, Chiloplacus, Acobeles, Eucephalous, Aphelenchus, Aphelenchoides, Tylenchus, Tetylenchus and Dorylaimus were found to be the dominant families/genera. Significant differences were found between months (p(0·01, n"156) in the genera of Cephalobus, Chiloplacus, Acobeles, Eucephalous, Aphelenchoides. Significant effects were also found between treatments (p(0·01, n"156) in the family of Rhabditidae and in the genera of Aphelenchus, Tylenchus, Tetylenchus and
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Table 1. Univariate analysis of variance (ANOVA) for soil conditions and nematodes under the canopy of the two shrubs, A. monosperma and R. raetam during the study period (April 1998–March 1999)
Variable
Month
Treatment
F-test
p value
F-test
p value
282·55 23·11 25·50 47·49 64·10 22·09
0·0001 0·0001 0·0001 0·0001 0·0001 0·0001
6·32 31·75 118·94 28·20 30·62 10·51
0·0025 0·0001 0·0001 0·0001 0·0001 0·0001
Nematode populationR Abundance BF FF PP OP
49·44 29·36 21·38 10·35 8·30
0·0001 0·0001 0·0001 0·0001 0·0001
51·21 29·73 29·77 9·73 8·17
0·0001 0·0001 0·0001 0·0008 0·0005
Ecological indexS (FF#BF)/PP F/B Modified F/B SI H MI
19·66 5·73 6·02 32·27 28·50 8·27
0·0001 0·0001 0·0001 0·0001 0·0001 0·0001
16·95 2·74 2·91 5·18 7·52 32·20
0·0001 NS NS 0·0070 0·0008 0·0001
Soil condition* SM OM EC Ca2> K> Na>
* SM, soil moisture; OM, organic matter; EC, electrical conductivity; cations: Ca2>, K>, Na>. R Absolute abundance: individuals 100 g\1 dry soil; trophic structure: BF, Bacterivores; FF, Fungivores; PP, Plant parasites; OP, Omnivores–predators. S (FF#BF)/PP, ratio of bacterivores and fungivores to plant parasites; F/B, fungivore/bacterivore ratio; Modified F/B, ratio of fungivores/(fungivores#bacterivores); SI, Simpson diversity index; H , Shannon Index; MI, modified maturity index. Values with p(0·05 were considered significant; NS, non-significant (n"156).
Dorylaimus. The densities of Rhabditidae, Aphelenchoides, Aphelenchus, Tetylenchus and Tylenchus exhibited positive correlations with soil electrical conductivity (p(0·01, n"156), and those of Chiloplacus, Rhabditidae, Aphelenchoides, Aphelenchus and Tetylenchus presented negative correlations with the contents of Ca2> and K> (p(0·05, n"156). However, a negative correlation was found between Eucephalous and soil electrical conductivity (p(0·01, n"156), and positive correlations were found between Eucephalous and the contents of Ca2> and K> (p(0·05, n"156). The total number of nematodes ranged between four and 377 individuals 100 g\1 dry soil. The total numbers of nematodes under A. monosperma and R. raetam reached maximum values on 18 February 1999 (119$33 and 298$79, respectively) (Fig. 1c). These values were not comparable to the peak values of soil moisture obtained on 22 February after a rainfall event (Fig. 1a). During the winter season (December 1998 and March 1999), the mean numbers of total free-living nematodes were found to be lower under A. monosperma in comparison to R. raetam (Fig. 1c). Significant differences
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Figure 1. Changes in (a) soil moisture, (b) organic matter and (c) total number of nematodes under the canopy of the two shrubs Artemisia monosperma and Retama raetam during the study period (April 1998–March 1999). ( Control; A. monosperma; R. raetam.)
were found between treatments (p(0·01, n"156) (Table 1). Positive correlations were found between total nematodes and soil moisture (p(0·01, n"156) and soil organic matter (p(0·01, n"156) (Table 3), and negative correlations were found between total nematodes and Ca2> content (p(0·01, n"156) and K> content (p(0·01, n"156) (Table 3). Bacterivores were found to be the most abundant group in this study (Table 2). The populations of bacterivores under A. monosperma and R. raetam exhibited similar distribution to those of total nematodes (Fig. 3a). Significant differences were found between treatments (p(0·01, n"156) in the bacterivore group (Table 1). Positive correlations were found between bacterivores and soil moisture (p(0·01,
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Figure 2. Changes in (a) soil electrical conductivity, (b) Ca2>, (c) K> and (d) Na> under the canopy of the two shrubs Artemisia monosperma and Retama raetam during the study period (April 1998–March 1999). ( Control; A. monosperma; R. raetam.)
n"156), soil organic matter (p(0·01, n"156), and soil electrical conductivity (p(0·01, n"156). There was a negative correlation between bacterivores and Ca2> content (p(0·01, n"120) (Table 3). The populations of fungivores under A. monosperma and R. raetam changed slightly during April and December 1998, and then fluctuated greatly from January to March 1999. The populations of fungivores during December 1998 and March 1999 were lower under A. monosperma than under R. raetam. (Fig. 3b). The maximum values of fungivores under A. monosperma and R. raetam were found on 18 February 1999 (Fig. 3b). Significant difference was found between treatments (p(0·01,
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Table 2. Nematode genera and families identified under the canopy of the two shrubs, A. monosperma and R. raetam, during the study period (April 1998– March 1999)
Trophic group
Family
Genera
c-p*
%R
Alaimidae Cephalobidae
Diploscapteridae Monhysteridae Plectidae Rhabditidae
Alaimus Acrobeles Acrobeloides Cephalobus Chiloplacus Eucephalobus Diploscapter —S Plectus —S
4 2 2 2 2 2 1 1 2 1
66·2 0·5 3·7 2·3 24·6 4·5 3·3 0·1 2·2 0·5 24·5
Anguinidae Aphelenchidae Aphelenchoididae
Ditylenchus Aphelenchus Aphelenchoides
2 2 2
19·1 0·3 7·8 11·0
Belonolaimidae Heteroderidae Longidoridae Pratylenchidae Tylenchidae
Tylenchorhynchus Heterodera Xiphinema Pratylenchus Tetylenchus Tylenchus
3
Dorylaimidae
Discolaimus Dorylaimus Leptonchus
4 4 4
Bacterivores
Fungivores
Plant-parasites 3 2 2
Omnivores–predators Leptonchidae
11·1 0·8 0·4 0·1 1·0 4·2 4·7 3·6 0·6 2·9 0·1
* Taxa are classified by trophic groups and their respective c-p value as defined by Bongers (1990) and Neher et al. (1995). R Mean relative abundance. S Not identified to genus.
n"156) in the fungivore group (Table 1). Positive correlations were found between fungivore populations and soil moisture (p(0·01, n"156) and soil electrical conductivity (p(0·01, n"156), and fungivores exhibited negative correlations with Ca2> content (p(0·01, n"156) and K> content (p(0·01, n"156) (Table 3). The populations of plant parasites under A. monosperma and R. raetam fluctuated slighty during the study period except the sampling under R. raetam on 18 February 1999 (Fig. 3c). Significant differences were found between treatments (p(0·01, n"156) in the plant parasite group (Table 1). Positive correlations were found between fungivore populations and soil moisture (p(0·01, n"156) and electrical conductivity (p(0·01, n"156), and fungivores exhibited negative correlations with Ca2> content (p(0·01, n"156) and K> content (p(0·05, n"156) (Table 3). Omnivores-predators were the least abundant group in this study. The densities of omnivores-predators under A. monosperma and under R. raetam fluctuated slightly,
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Table 3. Correlation coefficients between indices of nematodes and soil conditions under the canopy of the two shrubs: A. monosperma and R. raetam during the study period (April 1998–March 1999)
Index AbundanceR
SM
OM
EC
0·405**
0·241**
0·282
Trophic structureS BF 0·415** FF 0·530** PP 0·270** OP 0·273** Ecological indexA (FF#BF)/PP !0·087 F/B 0·151 Modified F/B 0·164 SI 0·001 H !0·073 MI !0·050
Ca2>
K>
!0·273** !0·159*
Na> 0·104
0·241** 0·282**!0·184* !0·020 !0·059 0·042 0·489**!0·291** !0·240** 0·015 0·054 0·380**!0·222** !0·191* 0·082 0·033 !0·021 !0·018 0·025 !0·164* 0·231** !0·110 !0·118 0·223** 0·342** !0·021
!0·145 0·235** 0·326** 0·007 0·084 !0·131 !0·142 !0·036 0·090 !0·138 !0·151 !0·043 !0·212** 0·070 0·167* !0·057 !0·234** 0·167* 0·288** !0·034 !0·480** 0·019 0·104* !0·148
* ** Correlation coefficients significant at p(0·05 and 0·01, respectively (n"156). R SM, soil moisture; OM, organic matter; EC, electrical conductivity; cations: Ca2>, K>, Na>. S Absolute abundance: individuals 100 g\1 dry soil; trophic structure: BF, Bacterivores; FF, Fungivores; PP, Plant parasites; OP, Omnivores–predators. A (FF#BF)/PP, ratio of bacterivores and fungivores to plant parasites; F/B, fungivore/bacterivore ratio; Modified F/B, ratio of fungivores/(fungivores#bacterivores); SI, Simpson diversity index; H , Shannon Index; MI, modified maturity index.
except for the sampling under R. raetam on 22 February 1999 which reached maximum value (20$9) (Fig. 3d). Significant differences were found between treatments (p(0·01, n"156) in the omnivore-predator group (Table 1). A positive correlation was found between omnivores-predators and soil moisture (p(0·01, n"156), and omnivores-predators exhibited a negative correlation with K> content (p(0·05, n"156) (Table 3). Ecological indices The ratio of bacterivores and fungivores to plant parasites [(FF#BF)/PP] under the two shrubs and control soil fluctuated greatly during the study period (Fig. 4a). During April and July 1998, the values of [(FF#BF)/PP] were higher under A. monosperma than under R. raetam. The minimum values of [(FF#BF)/PP] under both shrubs were found in October 1998 (Fig. 4a) before the rainy season. Significant differences were observed between treatments (p(0·01, n"156) in the [(FF#BF)/PP] ratio (Table 1). Positive correlations were found between [(FF#BF)/PP] ratio and soil organic matter (p(0·01, n"156), Ca> content (p(0·01, n"156) and K> content (p(0·01, n"156) (Table 3). The ratio of fungivores to bacterivores (F/B) under A. monosperma and R. raetam in our study site was found to be less than one (Fig. 4b). In the December 1998 and March 1999 samples, the values of F/B were found to be lower under A. monosperma than under R. raetam, where the F/B ratio under A. monosperma reached one peak in August 1998 (Fig. 4b). The maximum value of F/B under R. raetam was found on 22 February 1999.
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Figure 3. Distribution of nematode trophic groups under the canopy of the two shrubs Artemisia monosperma and Retama raetam during the study period (April 1998}March 1999). (a) Bacterivores; (b) fungivores; (c) plant parasites; (d) omnivores–predators. ( Control; A. monosperma; R. raetam.)
No correlations were found between the F/B ratio and soil conditions (p(0·05, n"156) (Table 3). The values of Modified F/B under A. monosperma and R. raetam exhibited a similar trend to those of F/B during the study period (Fig. 4c). No significant difference was found between treatments in the Modified F/B ratio during the study period (p(0·05, n"156) (Table 1). The Simpson diversity index (SI) under A. monosperma and R. raetam presented similar trends during July 1998 and January 1999, and the maximum values of SI under
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Figure 4. Variations of ecological indices [(a) (FF#BF)/PP; (b) F/B; (c) Modified F/B] under the canopy of the two shrubs Artemisia monosperma and Retama raetam during the study period (April 1998–March 1999). ( Control; A. monosperma; R. raetam.)
both shrubs were found in August 1998. The values of SI were found to be lower under A. monosperma than under R. raetam during September 1998 and February 1999 (Fig. 5a). Significant differences were found between treatments (p(0·01, n"156) in the values of SI (Table 1). Positive correlations were found between SI values and soil organic matter (p(0·01, n"156) and K> content (p(0·05, n"156), and a negative correlation was found between SI values and soil electrical conductivity (p(0·01, n"156) (Table 3). The values of Shannon index (H ) under A. monosperma and R. raetam presented similar trends to those of SI during the study period (Fig. 5b). Significant differences were found between treatments (p(0·01, n"156) in the values of H (Table 1).
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Figure 5. Variations of ecological indices (a) SI; (b) H ; (c) MI; under the canopy of the two shrubs Artemisia monosperma and Retama raetam during the study period (April 1998–March 1999). ( Control; A. monosperma; R. raetam.)
Positive correlations were found between H values and soil organic matter (p(0·01, n"156), Ca> content (p(0·05, n"156) and K> content (p(0·01, n"156). A negative correlation was found between H values and soil electrical conductivity (p(0·01, n"156) (Table 3). The values of modified maturity index ( MI) under A. monosperma and R. raetam similar trends, and fluctuated slightly during the study period (Fig. 5c). The values of MI during April 1998 and February 1999 were lower under both shrubs than under the control soil except for the sampling on 20 October 1998 under R. raetam (Fig. 5c). Significant differences were found between treatments (p(0·01, n"156) in the values of MI (Table 1). A positive correlation was found between MI values and
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K> content (p(0·05, n"156), and a negative correlation was found between MI values and soil electrical conductivity (p(0·01, n"156) (Table 3). Discussion In this study, yearly fluctuations in soil conditions such as soil moisture, organic matter, soil salinity, and soluble cations, have significantly influenced the temporal distribution of nematodes under the canopy of two desert halophyte shrubs. Significant differences were found between treatments for the soil environmental variables, total nematodes and the four trophic groups. The contents of soil moisture exhibited positive effects on the total nematodes and the four trophic groups during the study period; the results of our study supported that water is one of the major factors controlling biological activity in a desert ecosystem (Noy-Meir, 1974). Positive correlations were found between electrical conductivity and bacterivores, fungivores and plant parasites, and negative effects were found between Ca> content and total nematodes, bacterivores, fungivores and plant parasites. The K> content presented significantly negative effects on the total nematodes, fungivores and plant parasites. Omnivorespredators were sensitive to Na> content during the study period. The findings of our results that K> and Na> exhibited strong negative effects to soil free-living nematodes were comparable to those in the Dry Valley region of Antarctica reported by Freckman & Virginia (1989). During the study period, 20 nematode taxa in our study were found in the northern Mojave Desert ecosystem, which is higher than that (18 genera) reported by Freckman & Mankau (1986). The mean absolute abundance of total nematodes in our experimental site was 59 individuals 100 g dry soil\1, which was higher than that (48) reported by Liang et al. (2000) at the Avdat Research Farm, and lower than that (272) under the canopy of Z. dumosum observed by Steinberger & Loboda (1991) at the Avdat Research Farm, and also lower than that (912) obtained by Freckman & Mankau (1986) in the northern Mojave Desert ecosystem. Bacterivores were the most abundant trophic group and omnivores–predators were the least abundant group in both treatments and controls during the study period, averaging 66·2% and 3·6% of the nematode community, respectively. The mean relative abundance of omnivores–predators was lower than that found by Freckman & Mankau (1986) in the northern Mojave Desert ecosystem, which is classified as a ‘cold desert’ compared with the ‘hot’ Negev Desert. Low proportions of omnivores–predators in the nematode populations in the Negev Desert are indicative of a lower stability of the habitat. Various ecological indices were effective in comparing and assessing the responses of nematodes to disturbed soil environments (Freckman & Ettema, 1993). The ecological indices we tested were also useful in distinguishing nematode communities in this study, since significant differences were found both between months and between treatments in all ecological indices we used. Simpson’s diversity index, H and MI were sensitive to stressed soil environmental factors such as electrical conductivity and K> content. The F/B ratio reflects the structure of the microflora community. Bacteria and fungi are the primary decomposers directly affecting nutrient cycling and nutrient supply to plants (Ingham et al., 1985). Bacteria-based food webs exhibit higher decomposition rates than fungi-based webs (Porazinska & Coleman, 1995). The F/B ratio value in this study was 0·34, which was lower than that in agroecosystems (Freckman & Ettema, 1993). Our estimate (0·23) of the Modified F/B ratio was similar to that estimated for perennial crop (0·21) by Neher & Campbell (1994), and lower than that measured for asparagu (Asparagus spp.) (0·28) by Yeates et al. (1993). This means that there is a high population of bacteria in the sandy desert ecosystem, and a bacterial decomposition pathway was dominant (Hendrix et al., 1986). The mean value of (FF#BF)/PP in our
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study is 9·15, which was higher than the range (0·80–8·70) of values obtained by Wasilewska (1994) for meadow communities. The Shannon index (H ) gives more weight to rare species, and a higher index indicates greater diversity. The average H value in our experimental site was 1·53, which is lower than that (2·10) obtained by Freckman & Ettema (1993) in agroecosystems. The mean value of MI in our study was 1·85, which was comparable to the range (1·80–2·20) for meadows less than 4 years in Poland (Wasilewska, 1994), and lower than that (2·11–3·54) in New Zealand pastures (Yeates, 1994). Our results suggest that the seasonal fluctuations of the stressed soil environments in the northern Negev Desert significantly influenced the distribution of nematode communities. The proportions of nematode trophic groups and the values of ecological indices of nematode communities have been shown to be sensitive indicators of seasonal changes in nematode community structure under the canopy of the two desert halophytes. The total number and plant-parasitic nematodes in our investigation were lower compared with other studies in desert soil ecosystems. There are two reasons. Firstly, the sampling depth of 0–10 cm is very shallow and plant-parasitic nematodes are distributed in deep-rooting desert ecosystems (Freckman & Virginia, 1989); secondly, Baermann funnel technique tends to favor extraction of bacterivous nematodes, the dead and inactive nematodes are not extracted from the soil samples. In order to completely understand the temporal changes in nematode community structure in the desert ecosystem, further studies should be considered at the proper sampling depths and using efficient extraction techniques for nematodes. This project was supported by a Fred and Barbara Kort Sino-Israel Postdoctoral Fellowship to Dr Liang Wenju. The authors wish to express their appreciation to Mrs Ginetta Barness, Mr Amir Alon, Ms Irit Lavian and Mrs Yocheved Pinhasi-Adiv for useful suggestions during the study and for technical assistance. We also appreciate the helpful comments of an unknown reviewer.
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