effect of irrigation on nematode population dynamics and activity in desert soils

Biol Fertil Soils (1987) 3:3-10 Biology and Fertility

of S o i l s © Springer-Verlag 1987

Effect of irrigation on nematode population dynamics and activity in desert soils*

D.W. Freekman 1, W.G. Whitford 2, and Y. Steinberger 3

1 Department of Nematology, University of Calfornia, Riverside, CA 92521, USA 2 Department of Biology, Box 3AF, New Mexico State University, Las Cruces, NM 88003, USA 3 Department of Biology, Bar Ilan University, Ramt Gan, Israel

Summary. The nematode community in litter and soil was examined for a year in the Chihuahuan desert, before and after supplemental rainfall application. Pro- portions of nematode-active or anhydrobiotic forms and population densities were determined for 3 treatments: control (natural rainfall), a single, large (25- ram) monthly irrigation pulse, and 4 smaller (6-mm) irrigations spaced at weekly intervals. In fitter the greatest nematode abundance was in the 6 mm week -1 treatment (48 nematodes 20 g-1 fitter). Bacteriovores and fungivores accounted for approximately 95% of the numbers and biomass in all treatments. In soil, water amendments had no significant effect (P < 0.05) on annual mean densities of total nematodes, fungivores, bacterivores, or omnivore predators. Phytophage densities were greater on both irrigation treatments, with highest densities (9268 m -z) in the 6 mm week -1 soils, which was 5.9% of the total soil nematode density. Total densities of individual trophic groups were not significantly different before or after rainfaU. Soil nematode densities fluctuated independently with trophic group, month, and season. Bacterial feeders and omnivore predators were the largest contributor to total soil nematode density and biomass. Prior to irrigation, there were no differences in the percentage of anhydrobiotes on the three treatments. Anhydrobiotes decreased after irrigation in all treatments, and were significantly lower in soils of the larger, monthly irrigation. Nematodes were inactive (anhydrobiotic) and decoupled from decomposition processes when soil water matric potentials reached -0.4 MPa.

*Dedicated to the late Prof. Dr. M.S. Ghilarov Offprint requests to: D.W. Freckman

Key words: Nematode community - Chihuahan desert - Irrigation - Nematode extraction - Anhydro- biosis

Nematodes are ectothermic, aquatic animals and are confined to the water films surrounding soil particles. Their ability to survive desiccation in plant parts for 30- 40 years and in soils for days to years at any stage of their life cycle has been the focus of many ecophysiological investigations (Demeure et al. 1979a, b; Freck- man et al. 1980; Demeure and Freckman 1981; Freck- man and Womersley 1983). Nematodes adapt to dehy- dration or freezing by entering into an inactive state ter- med anhydrobiosis or cryptobiosis, which is immediately reversible by reduction of the environmental stress.

Coventional wisdom holds that biological processes in deserts are regulated by rainfall (Noy-Meir 1973). Noy-Meir hypothesized that the pulse of biotic activity following rainfall would rapidly deplete the free energy compartment of organisms, leawing a small amount of energy held in reserve in the form of spores or anhydrobiotic or resistant stages. Previous studies in the Chihua- huan Desert have shown that 80 % of the nematodes in surface litter became anhydrobiotic 6 h after applying a simulated 25 mm rainfall, whereas nematodes in slower drying soil under the litter took 4 days to reach a similar level of anhydrobiosis (Whitford et al. 1981). While in anhydrobiosis the nematodes are essentially decoupled from the decomposition processes.

Studies of responses of desert soil nematodes to wetting and drying have been short term, i.e., 1 month or less (Whitford et al. 1981 ; Steinberger et al. 1984). We have no information on seasonal variation in short- term responses, nor information on the effects of

4 D.W. Freckman et al.: Nematodes in desert soils

varying levels of wetting and drying on the population dynamics of soil nematodes. As part of a year-long collaborative study of the effects of supplemental precipita- tion on decomposition processes (Whitford et al. 1985) and soil microflora and microfauna, we made a detailed study of the desert soil nematode community before and after water application for 1 year. We hypothesized that a single, large (25-mm) irrigation pulse should have a greater effect on decomposition, nematode densities, and nematode activity than four smaller (6-ram) irrigations spaced at weekly intervals. Both of these treatments should result in higher decomposition rates and more abundant soil fauna and microflora than a natural rainfall (Whitford et al. 1985).

Materials and Methods

These studies were conducted at the base of an alluvial plain in New Mexico at an elevation of 1000-2000 m. The soils are deep sandy loams with a calcium carbonate deposition layer (caliche) at approximately 70 cm depth. The dominant vegetation is a cover of cresote bush, Larrea tridentata, with scattered mesquite, Prosopis glandu- losa, along drainage channels. The average annual rainfall is 211 _+ 77 mm, with 70% of the precipation occurring in late summer. Summer maximum temperatures reach 40°C and winter temperatures regularly fall below 0°C.

The experimental design has been described in detail (Whitford et al. 1985) and is briefly described here..L, tridentata shrubs of approximately the same size were chosen: 70-100 cm in height and 100 crn canopy diameter. All leaf litter was cleared from under the canopy of the shrubs. Twenty-gram units of air-dried L. tridentata leaves were confined in aluminum window screen cylinders that were fastened to the soil surface under the shrub canopy. Rainfall amendments were provided by a sprinkler irrigation system situated at the ends of each 10 × 25 m plot that provided water above the plants. All plots received natural rainfall plus the following supplements: (1) control (no added water), (2) weekly applications of 6 mm water, and (3) monthly applications of 25 mm. There were three plots per treatment, with five replicates per treatment. The source of water for the irrigated plots was weft water collected in a concrete holding pond. The water had an electrical conductivity of 0.8 mmhos.cm -3 with approximately 100 mg.1-1 NaHCO3 and NaCO3, and 200 mg.1-1 NaC1. Soil moisture tension was measured at 5 cm for each plot on each sampling date with Wescor soil psychrometers, and gravimetric soil moisture was determined from each soil sample. Soil moisture tension release curves arid texture analyses were determined from soil collected at control plots. Soil temperatures were recorded for each treatment. Litter was carefully removed and soil cores (4.5 cm diameter and 10 cm deep) were collected immediately beneath the litter. Samples were removed from each plot between 0600 h and 0800 h, immediately before and 3 days after each simulated rainfall. Litter and soil cores were placed in plastic bags and shipped to the University of California, Riverside, for nematode extraction.

Litter extraction. Because most nematode extraction methods were developed for removing nematodes from soil, two methods, the mist chamber (Southey 1970) and a modification of the Coolen technique (Coolen 1979), were tested to determine the most efficient technique for extracting active nematodes from litter. L. tridentata leaf litter was weighed into 20-g aliquots and placed in 20 individual plastic bags. One milllter of water containing 1000 _+ 100 Aphelen- chus avenae, a fungal feeding nematode, was sprayed into each bag, the bag sealed, and the nematodes and litter gently mixed. Litter from each bag was poured into a food processor and processed

gently for 30 s. For the mist chamber method, litter from each of 10 bags was placed on the mist chamber and nematodes removed daily for 7 days.

Nematodes were extracted from the remaining ten bags using a modified Coolen technique, originally developed to extract nematodes from root fragments. This method consisted of the following: litter was blended with 200 ml water, poured into a centrifuge tube containing 2.5 ml kaolin powder, stirred for 30 s, and centrifuged at 1500 g for 4 min. Water was decanted, 1 M sucrose solution added to make a volume of 200 ml, the solution stirred for 30 s, centrifuged for 4 min at 1500 g, poured onto 5-[xm Coolen sieves, and left for 5min. The stoppers were removed from the sieves, the sieves sprayed with a fine water mist, and the nematode-water solution poured into a 150-ml beaker. Two drops of Separan (Dow Chemical Co) per beaker were added, and the solution was stirred, allowed to settle, and poured through a 500-mesh sieve.

The differences between the two techniques were significant (P < 0.01), with a mean recovery of 850 nematodes for the Coolen method and 420 for the 7-day total of the mist chamber. The nematodes in the samples included both bacterial feeding nematodes already present in the litter and the fungivore A. avenae. The mist chamber technique extracted a greater percentage of bacterial feeders, most of which were juveniles. Further replications of the Coolen technique with A. avenae indicated an extraction efficiency of 75%. Because the mist chamber extracted larger numbers of juveniles which may have hatched from eggs over the 7-day period, and because the fungivorous trophic group was not well represented by this method, the Coolen technique was used to extract nematodes from litter. All densities reported are corrected for extraction efficiency, and were calculated on the basis of numbers extracted from 20 g litter.

Soil extraction. Nematodes from soil samples were extracted by two techniques. To determine taxonomic identification, density and affiliation with trophic groups, one-half of the soil samples were processed by the modified sugar flotation technique (Freckman et al. 1975). The nematode genera and trophic groups were: bacterivores - Acrobeles, Acrobeloides, Alaimus, Cephalobus, Panagrolaimus, Plectus, Rhabditida; fungivores - Aphelenchus avenae, Aphelen- choides, Ditylenchus, Stictylus; omnivore predators - Dorylaimus, Prismatolaimus, Pungentus; plant feeders - Quinsulcius, Paraty- lenchus, Tylenchorhynchus. Nematode numbers were expressed as numbers m -2 at 0-10 cm depth, and were corrected for an extraction efficiency of 75% (Freekman et al. 1975). Bulk density of the soil (1.62 g/cm 3) was considered in calculations of the nematode density. Analysis of variance and studentized range test (Tukey's) were per- formed on the transformed (log [X+ 1]) numbers of nematodes.

Biomass was determined by measuring the lengths and widths of >100 nematodes per trophic group (Freckman 1982), and average individual weight per trophic group was calculated according to Andrassy (1956).

To assess the level of activity of the nematode population (anhydrobiotic or active)nematodes were extracted from the remaining soil by the anhydrobiotic technique (Freckman et al. 1977) and the percentage of coiled and straight nematodes determined. Coiled nematodes were considered indicative of the inactive, anhydrobiotic state (Demeure et al. 1979a).

Results and discussion

Litter nematodes

O n l y resu l t s b a s e d o n t h e m o n t h l y analys is will b e

d i scussed h e r e . R e s u l t s o f t r e a t m e n t e f fec t s o n a n n u a l

m e a n n e m a t o d e a n d t r o p h i c g r o u p d en s i t y in l i t ter

D.W. Freckman et al.: Nematodes in desert softs 5

Table 1.4. Significant effects in nematode numbers in samples taken before and after sprinkler irrigation (0 ram, 6 m m . week "1 , and 25 ram- m o n t h "~ ) in desert surface litter. Significant effects were demonst ra ted in the mon t hs listed. A. (-) indicates a greater nematode abundance before irrigation. (+) indicates an increased abundance after irrigation

Fungi- Bacteri- Omnivore Phyto- Total vores votes predators phages

July *a/_ */_ */_ NS/ ** / - August **b/_ */_ NS/ NS/ NS/ September **/+ NS/ NS/ NS/ NS/ October ** / - NS/ ***c/+ ***/+ NS/ April ** / - ** / - NS/ NS d ** / -

Table lB. Significant increases in nematode abundance in irrigated t rea tments (6 m m . w e e k "1, and 25 m m - m o n t h "1) relative to the control (0 mm) in desert surface litter

August ** * NS NS * November * NS NS NS NS January * * NS NS * February ** NS NS NS NS April ** *** NS NS *** June NS * * NS *

a* = P < 0.05 b** = P < 0.01 c*** = P < 0 . 0 0 1 d NS = Not significant

have been presented with other soil biota discussed in this study (Whitford et al. 1985). Briefly, the 6 mm week -1 treatment had a greater nematode abundance (48 nematodes 20g q litter). Both abundance and biomass were dominated by two trophic groups, bacterial feeders and fungal feeders.

Total nematode numbers and trophic group abundance were not affected (P < 0.05) by irrigation or time of sampling (before or after irrigation) in June or Feb- mary. There were only a few treatment effects for the other months (Table 1A). Abundance was greatest before water application in: July, August, October, and April for fungivores; July, August, and October for bacterial feeders; July for omnivore predators, and July and April for monthly totals. Abundance was greater (P < 0.05) after irrigation only 3 times: in Sep- tember for fungivores, and in October for omnivore predators and phytophages. There was no significant or consistent pattern in numerical responses through time among trophic groups.

In general, when significant differences in nematode numbers occurred with irrigation, the 6 mm week q treatments had greater (P < 0.05) densities than the control, but not significantly different from the 25 mm water month -1 amendment (Table 1B). This effect occurred in August and November for fungal feeders, August, January, and June 1982 for bacterial feeders, June 1982 for omnivore predators, and in August, Jan- uary, April, and June 1982 for monthly totals. In

January, February, and April, fungivore abundance increased significantly (P < 0.05) from the control to the 25 mm month q to the 6 mm week -1 and in April for bacterial feeders (220 nematodes 20 gq litter).

Sampling was done prior to irrigation and 3 days after irrigation. The water content of the litter was affected by drying during the post-watering interval and was variable throughout the study (Whitford et al. 1985). Populations of soil organisms in surface litter are a subset of the soil populations. Nematodes could enter the fitter by migration from the soil or as airborne particles (anhydrobiotic forms). The placement of cylinders under the shrub canopy probably reduced airborne additions of nematodes. Populations in the litter may thus reflect seasonal migrations from soil into litter during wet periods when temperatures were favor- able (Whitford et al. 1985). Vertical migration into the surface litter could therefore be a function of the duration of saturated conditions at the soil-litter interface.

Soil nematodes

Water amendments had no significant effect (P < 0.05) on annual mean densities of total nematodes (Fig. 1A) fungal feeders (Fig. 1B), bacterial feeders (Fig. 1E), or omnivore predators (Fig. 1C). Phytophages (Fig. 1D) significantly increased (P < 0.05) in both irrigation treatments, with highest densities (9300 m -2) on the 6 mm week -1 treatment. There were no significant differences in total mean or individual trophic group densities before and after irrigation. Since Larrea has a well-developed surface lateral root system in the upper 10 cm, the increase in phytophages may be in response to a stimulation of root growth and root activity from the irrigation.

Nematode densities fluctuated with month. Peaks for mean total densities occurred in January 1982 for both the control and 25 mm month -1 irrigated plots (530 400 m -2 and 392 900 m -2, respectively), and in Oc- tober (553 400 m -2) on the 6 mm week -1 irrigated plots (Fig. 1A). Soil temperatures (Fig. 2) were low during January and soil moisture high (Fig. 3). The groups mainly responsible for the peak in January, bacterial feeders and fungivores, had their highest densities. In contrast, omni;eore predators had their lowest density (3 700 m -2) for the year (P < 0.05) in January. Fungi- vores (Fig. 1B) decreased (P < 0.05) in both October 1981 and February 1982. Phytophage abundance peak- ed in December (14 800 m -2) and was lowest in No- vember (300 m -2) (Fig. 1D).

Bacterial feeders were the largest contributors to mean total density in all treatments; however, their biomass and percentage contributions to total biomass were greatest only in the 25 mm month -1 water amend-

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ment (Table 2). Omnivore predators were the other large group, contributing 56% of the total biomass in the control plots. Biomasses of both bacterial feeders and omnivore predators were higher in the control soils than in the 6 mm week -1 treatment soils• The percentage of bacterial feeders of the total biomass increased with increasing water amendments, whereas the percentage of omnivore predators decreased with increasing water. The contribution of phytophages to total density was very small (0.5%) in the control, higher in the 6 mm week -1 treatment (5.9%), with a corresponding significant increase in phytophage biomass (Table 2).

Using a 100-year data base of rainfall and ambient air temperatures at the Jomada Experimental Range,

Cunningham and Conley (1985 personal communica- tion) identified three distinct seasons for the Jomada. They were: warm-wet - July, August, September, Oc- tober; cold-variable - November, December, January, February; and warm-dry- March, April, May, June. To determine if there were any seasonal effects of the rainfall treatments on nematode densities, data were analyzed on a seasonal basis.

In the warm-wet season, total nematode densities were greater (P < 0.05) on the control and 6 mm week -1 irrigated treatment on the 25 mm month -1 amendment (Fig. 4A). The higher irrigation treatment may have leached nutrients below 10 cm depth, causing lower nematodes densities, although no treatment differences were noted in microflora (Parker et al. 1984;

D.W. Freckman et al.: Nematodes in desert soils 7

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Table 2. Mean numbers, percent of the population, biomass and percent of the total biomass of nematodes (104 • m -2) in non-irrigated control (0 mm) and irrigated soils (6 mm' week -1 , 25 mm - month -1 )

0 mm 6 mm 25 mm

No. % Biomass a (%) No. % Biomass a (%) No. % Biomass a (%)

Fungivores 0.11 (1) 0.27 (1) 0.18 (1) 0.20 (0.4) 0.13 (1) 0.14 (0.30) Bacterialfeeders 8.62 (81) 8.96 (42) 10.53 ( 6 8 ) 25.29 (48) 11.50 (77) 2 7 . 8 2 (59.00) Omnivore predator 1.86 (17) 11.88 (56) 3.95 ( 2 5 ) 25.25 (48) 2.94 (20) 18 .79 (40.00) Phytophages 0.05 (1) 0.06 (1) 0.93 (6) 1.10 (2) 0.29 (2) 0.30 (0.60) Total 10.64 21.17 15.59 51.84 14.91 47.05

a mg × 10 -4 fresh weight

Whitford et al. 1985). There were no differences in fungivore densities with season, in contrast to the monthly analysis. The two trophic groups affected by the irrigation treatments during the warm-wet and cold-variable season, phytophages (Fig. 4D) and omnivore predators (Fig. 4C), had varying responses. Phytophagous nematode densities were greater (P < 0.05) in the warm-wet season on the irrigated plots and omnivore predators significantly lower on the 25 m m month -1 plots, similar to total densities (P < 0.05). In October (warm-wet) the phytophagous nematode population was predominantly (70%) Paratylenchus juveniles. These two trophic groups also showed differences in the cold-variable season, with omnivore predators having their lowest density of the three seasons and phytophages increasing slightly. Both phytophages and omnivore predator densities were significantly higher on the irrigated plots during the cold-variable season. Soil moistures fluctuate less during these months (Fig. 3) (Schlesinger et al. 1986) and soil temperatures are low (Fig. 2). Bacterial feeders had higher densities than the other groups on the irrigated plots (Fig. 4E) in the warm-dry season, but, in general, their

densities were constant from season to season, as were microfloral densities (Whitford et al. 1985).

Anhydrobiosis

There were no significant differences in the percentage of anydrobiotic (inactive) nematodes (mean of the year) in the three treatments (Fig. 5A), which differed from 8-month results (Freckman and Womersley 1983), where the percentage in the control was significantly higher. There was a significant decrease in anhydrobiotes after water appfication (Fig. 5B). There were no differences in anhydrobiotes prior to irrigation in the three treatments (Fig. 5C). However, after rainfall amendments, anhydrobiotic nematodes decreased significantly (Fig. 5B) on all treatments, with the greatest reduction occurring in the monthly water amendment treatment (Fig. 5C). Anhydrobiotes on all treatments followed a similar monthly pattern throughout the year, with reductions (P < 0.05) occurring in August (3% anhydrobiotic) and January and December (17% and 23% anhydrobiotic), usually associated with in-

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creased soil moisture and natural rainfall. There were few differences between percentage anhydrobiotic nematodes taken before and after irrigation of the control and 6 mm week -1 treatments during the year. How- ever, the percentage of anhydrobiotes was significantly reduced (P < 0.05) in the 25 mm week -1 plots following irrigation.

The monthly means of percentage anhydrobiotes from all 3 treatments (control, 6 mm week -1, 25 mm month -1) were correlated with monthly means of gravimetric soil moisture (percentage oven dry weight) before (r = -0.639) and after (r = -0.861) irrigation and with soil moisture tension at 5 cm, before (r = -0.327) and after (r = -0.746) irrigation. Further analysis of the relationship between soil moisture and anhydrobiotes was determined using gravimetric soil moisture data

because: (1) gravimetric (percentage) soil moisture data were not significantly correlated with soil moisture tension before water amendments (r = -0.599), (2) soil moisture tension is variable and is difficult to measure accurately at lower (wet) moisture tensions (Campbell 1972), (3) data from psychrometers were based on point measurements within plots rather than near actual samples, and (4) data for anhydrobiotes for all combined data points from before and after sprinkler irrigation for each of the 3 treatments correlated more closely with gravimetric soil moisture (r = -0.810) than with soil moisture tension (r = -0.461).

Examination of the percentage of the anhydrobiotes plotted against gravimetric soil moisture (Fig. 6) indicates that there was considerable variation among data means. By applying a method analogous to that pro-

D.W. Freckman et al.: Nematodes in desert soils 9

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posed by Cate and Nelson (1971), however, a general moisture "threshold" appears to exist at about 4.7%, above which less than 40% of the nematodes are anhydrobiotic. Below a soil moisture content of 4.7%, about 60% of the nematodes are in anhydrobiosis.

The soil type was a loamy sand (80% sand, 13% silt, 6% clay). Soil moisture release curves for the soil indicate that a soil moisture of 4.7% would correspond with approximately -0.4 MPa soil water matric potential (Schlesinger at al. 1986). Demeure et al. (1979a), using a bacterial feeding nematode Acrobeloides spp. isolated from the Chihuahuan desert, determined that coiling and the physiological state of anhydrobiosis started long before the soils dried to a water potential

of -1.5 MPa. In laboratory experiments, 60% of the nematodes were anhydrobiotic at -0.4 MPa or 4.0% soil moisture content in a loamy sand (92.4% sand, 3.9% silt, 3.7% clay) soil. Previous estimates indicated that 90%-95% of the nematodes in desert soils were anhydrobiotic at soil moistures of about 2.5% (Freck- man 1978; Freckman and Mankau 1986).

A linear regression of anhydrobiotes plotted against soil moisture (Fig. 6) indicates 68% of the nematodes will be anhydrobiotic at 0% moisture (y = 68.0-6.01x, r = -0.812). Transformation of the percentage anhydrobiotic nematodes to arcsin(%) gave the same correlation, with a y axis of 63% anhydrobiosis for the nematodes. These values are much lower than the 98% coiling at -0.6 MPa results obtained on a pressure plate in laboratory experiments (Demeure et al. 1979a), but can be explained by the lack of a constant soil moisture throughout the soil profile in the field. Nematodes would be influenced by the varying soil moistures occurring in pockets of soil throughout the soil profile and would therefore respond with varying degrees of activity. Therefore, it appears that even in driest soils some nematodes can participate in mineralization processes. In particular, underneath the canopy of Larrea, where biological activity and soil type gradients are variable, some (about 40%) of the nematodes are active in soil microsites, even at soil moisture contents below 4.7%.

This does not imply that nematodes are usually inactive in desert soils. Techniques to measure the actual duration of the anhydrobiotic state are inade- quate. Nematode activity could occur more frequently in the soil surface when the combination of early morn- ing relative humidity and cooler temperature could influence a diurnal anhydrobiotic response (Simons 1973). However, Whitford et al. (1981) found no evi- dence of nematode diurnal activity at the time intervals examined (4 h). Detection of a diurnal anhydrobiotic response may require better technology. Feeding during each period of diurnal activity might be necessary as laboratory studies showed repeated induction of anhydrobiosis without feeding resulted in decreased nematode survival (Demeure et al. 1978).

Nematode activity in the surface horizon during the winter may be greater than in lower depths or than in isolated soil microsites during the summer. During winter months, soil water matric potentials are higher and more homogeneous due to lower evapotranspira- tion (Schlesinger et al. 1986).

The interaction of fluctuating daily minimum and maximum soil temperatures with soil moisture (Fig. 3) would affect nematode activity, reproduction, and thus decomposition. Laboratory studies (Freckman, unpub- lished) indicate Acrobeloides spp. has a 12-day generation time at 18°C and a 4-day generation time at 33 °--

10 D.W. Freckman et al.: Nematodes in desert soils

36°C with adequate moisture. Although anhydrobiosis interrupts the life cycle, each life stage responds quickly to adequate soil moisture and temperatures, begins grazing on microflora and microfauna, and in- fluences decomposition.

These results (Fig. 6) can be used to predict when nematodes are involved in decomposition processes in desert soils and indicate that nematodes actively partici- pated in soil processes when: (1) rainfalls of approximately 25 mm occur, and (2) the soil water matric potential exceeds -0.4 MPa.

This information on nematodes, combined with other results from this collaborative study (Parker et al. 1984; Whitford et al. 1983, 1985), indicate that in the Chihuahuan desert rainfall pulses are not as important in triggering decomposition as previously thought. Even though microfloral food sources were present in stable densities, the nematode densities were lower in surface litter, and in soil were more responsive to water matric potentials and temperature effects (Whitford et al. 1985). To understand how rates of decomposition are controlled on a wide geographical scale in desert ecosystems, further manipulation of the soil environmental factors that induce ecophysiological responses in the desert soil biota are necessary.

Acknowledgments. We thank Floyd Lane and Robin Chapman for plot setup and sample collection, Carol Toll and Mary Schram for their work with the nematodes, and C. Huszar for statistical advice. This research was funded by grants BSR 821539 and BSR 8215357 from the Ecosystems Division of the U.S. National Science Founda- tion.

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Received September 26, 1985

effect of irrigation on nematode population dynamics and activity in desert soils

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