Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue11-12.oct2000:

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Responses of trophic groups of soil nematodes to residue

application under conventional tillage and no-till regimes

Shenglei Fu*, David C. Coleman, Paul F. Hendrix, D.A. Crossley Jr.

Institute of Ecology, University of Georgia, Athens, GA 30602-2202, USA

Accepted 13 April 2000

Abstract

A laboratory and a ®eld study were conducted to monitor the increase in numbers and14C uptake of dierent trophic groups of soil nematodes in response to residue addition and to examine the relative importance of bacterivorous and fungivorous nematodes in conventional (CT) and no-till (NT) agroecosystems. In general, soil nematode numbers increased more rapidly in response to residue addition and became much more abundant (greater than ®ve-fold) under laboratory conditions than in the ®eld. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivorous nematodes under both CT and NT regimes in the laboratory and ®eld studies. A depth eect was observed in NT, but not in the CT treatment; this re¯ected the vertical residue distribution in both tillage regimes. Soil nematodes were more abundant under NT than under CT in the ®eld. The same pattern was observed at the beginning of the laboratory study but it reversed later. The ratios of fungivorous-to-bacterivorous nematodes (FN-to-BN) were not signi®cantly dierent between CT and NT treatments at the beginning of the experiment. They were very low (less than 0.2) in both tillage regimes, indicating that bacterivorous nematodes were relatively more important than fungivorous nematodes in both tillage agroecosystems. However, the FN-to-BN ratios increased with time after residue decomposition started, particularly in the CT treatment. This suggested that the relative importance of fungivorous nematodes increased with the progress of residue decomposition. It was more pronounced in the CT treatment during the short period after residue application. In both the laboratory and ®eld studies, the14C speci®c activity of soil nematodes and the ratio of 14C bound in nematode biomass to total 14C decayed in the experiment (reported elsewhere) were signi®cantly higher under CT than under NT, suggesting that soil nematodes use carbon more eciently under CT than under NT. No signi®cant dierence of 14C speci®c activity of soil nematodes was found between the two depths under CT in both the studies; however,14C speci®c activity was signi®cantly higher in the 0±2.5 cm than in the 2.5±5.0 cm layer under NT in the laboratory study.72000 Elsevier Science Ltd. All rights reserved.

Keywords:Trophic groups; Soil nematodes;14_{C speci®c activity; Conventional tillage; No-till}

1. Introduction

Nematodes are one of the most abundant groups of soil invertebrates. More than four out of ®ve metazoan individuals on earth are nematodes, often reaching sev-eral millions per square meter (Bongers and Bongers, 1998). Although the contribution of soil nematodes to

total soil respiration is very low, soil nematodes are believed to have profound eects on soil processes through their in¯uence on the composition and activity of soil micro¯ora (Petersen and Luxton, 1982). Several microcosm studies have shown that the presence of soil animals (e.g. nematodes) can directly aect the biomass and activity of the microbial community through feeding on fungi and bacteria (Bardgett et al., 1993a, 1993b; Ferris et al., 1997). Soil nematodes are signi®cant regulators of residue decomposition and nutrient release in natural ecosystems through their high turnover rates and their interactions with

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* Corresponding author. Department of Environmental Studies, University of California, 339 Natural Sciences 2, Santa Cruz, CA 95064, USA. Tel.: +1-831-459-3685; fax: +1-831-459-4015.

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¯ora (Santos et al., 1981; Coleman et al., 1984; Parker et al., 1984; Ingham, et al., 1985; Moore et al., 1988). Based on model calculations, approximately 30% of the annual N mineralization in Lovinkhoeve agricul-tural soil was due to the contribution of bacterivorous nematodes (de Ruiter et al., 1993).

Trophic structure is a functional classi®cation that contributes to understand the structure of the nema-tode community, and how each group aects the trans-fer of matter or energy in the ecosystem (Freckman and Caswell, 1985). Many studies have focused on the nematode response to organic amendments and pollu-tants, seasonal cycles, and the spatial distribution of dierent trophic groups of soil nematodes in natural and agricultural ecosystems (Stinner and Crossley, 1982; Parmelee and Alston, 1986; Ettema and Bongers, 1993; Freckman and Ettema, 1993; Ruess, 1995). `Colonizers', with high reproduction rates and short life cycles, are thought to respond rapidly to high nutrient availability; and `persisters', with low repro-duction rates and longer life cycles, are believed to be more sensitive to soil disturbance (Bongers, 1990).

Any soil disturbance can aect soil nematode trophic structure and total abundance. In agroecosys-tems, tillage is the major disturbance to soil and it causes the redistribution of plant residue and soil or-ganic matter, subsequently changing microbial struc-ture and nematode trophic strucstruc-ture. Parmelee and Alston (1986) found that bacterivorous nematodes were more abundant in conventional tillage (CT) than in no-till (NT) plots over an annual cycle, whereas fun-givorous nematodes were more abundant in NT plots during the dry summer cropping season, but more nu-merous in CT during winters. Beare et al. (1992) con-cluded that fungivorous microarthopods were relatively more important in determining litter C losses in NT, while bacterivorous nematodes had a greater in¯uence in CT. Bardgett (1998) also reported that fungivores were twice as abundant in organically man-aged grassland systems as in conventionally manman-aged soils. However, the detrital food web at Lovinkhoeve was dominated by bacteria and bacterivores under both conventional and integrated farm management (Brussaard et al., 1990; de Ruiter et al., 1993). Stinner and Crossley (1982) found that total numbers of nema-todes and free-living nemanema-todes (bacterivores and fun-givores) were not signi®cantly dierent in CT and NT, but phytophages were signi®cantly dierent in the two tillage treatments. The dierences between the ®ndings of various studies indicate that the dynamics of trophic groups of soil nematodes under dierent tillage regimes need more detailed investigation. The import-ance of a bacterial-based food web versus a fungal-based food web in dierent tillage regimes needs to be con®rmed.

Our objectives were to monitor the response of

dierent trophic groups of soil nematodes to residue decomposition and to test the relative importance of bacterivorous and fungivorous nematodes in CT and NT agroecosystems. In addition, 14C speci®c activity of soil nematodes was determined to compare the car-bon use eciencies under dierent tillage regimes. A laboratory and a ®eld study were conducted separately for these purposes.

2. Materials and methods

2.1. Site description

The study was conducted at the Horseshoe Bend ex-perimental area, near Athens, GA. The soil is charac-terized as a well-drained moderately acidic sandy clay loam (Thermic Kanhapludult). Annual mean minimum and maximum soil temperatures were 8.3 and 19.38C for CT, and 9.5 and 17.58C for NT plots. The research plots in this experiment consisted of six alternating CT and NT plots, collectively occupying approximately 0.1 ha. These plots have been managed as CT and NT since 1984. In CT, the soil was moldboard plowed, disked and then rotary tilled before planting. In NT, the soil remained undisturbed except for a surface slit cut at the time of planting. Maize (Zea mays) was grown as a summer crop, following wheat (Triticum aestivum) or clover (Trifolium incarnatum) cover crops during winter months (Hendrix et al., 1986; Parmelee and Alston, 1986; Beare et al., 1992).

2.2. Experimental

2.2.1. Laboratory study

On 16 March 1997, intact soil cores were taken from the CT and NT ®eld plots. Each soil core from the CT ®eld plot was fragmented by hand and visible residues were picked out by hand. Then 1.0 g of 14C labeled corn leaf litter was mixed with the soil (equiv-alent to 509 g mÿ2_{). The corn leaf litter had been cut}

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2.2.2. Field study

On 6 April 1998, the CT plots were cultivated by hand using a spade and visible old residues were removed. PVC tubes (10 cm in diameter, 10 cm in height) were placed into the soil at a depth of 5.0 cm. Then 4.0 g of 14C labeled corn leaf litter were incor-porated into the soil in each core (equivalent to 509 g mÿ2_{). Old surface residue was removed from the NT}

plots and PVC tubes were set into the soil at a depth of 5.0 cm, and 4.0 g of14C labeled corn leaf litter was applied on the soil surface inside each tube. We sampled three CT and NT soil cores on each sampling date.

2.3. Nematode extraction and identi®cation

Soil cores were cut into two layers as 0±2.5 and 2.5± 5.0 cm, and at each layer, approximately 50 g of moist soil was obtained and used for nematode extraction. Soil nematodes were extracted using the Baermann funnel method (McSorley, 1987). After ®xation in 4% formaldehyde solution, nematodes were counted under an inverted microscope. To identify the nematodes, we evenly divided the microscopic ®eld into four sections making a cross mark under the Petri dish. Then 50 nematodes were randomly selected in each section for identi®cation such that a total of 200 nematodes per sample was identi®ed into ®ve trophic groups: bacteri-vores, fungibacteri-vores, phytophages, predators and omni-vores (Yeates et al., 1993). The whole sample was identi®ed when total numbers of nematodes were less than 200.

We measured 1000 randomly selected individual nematodes for width and length. Nematode biomass was then calculated according to Andrassy's (1956) formula and converted to dry weight assuming a dry-mass content of 25% (Yeates, 1979). Nematode samples were ®ltered with glass micro®bre ®lters (25

mm in diameter, Cat No. 1820-025, Whatman Inter-national). Each ®lter with nematodes was then put into a 20 ml vial. Nematodes were then digested with 1 ml of Scintigest (Fisher Scienti®c, Fair Lawn, NJ) for at least 48 h. The samples were then diluted with 1 ml deionized water and neutralized with 1 ml acetic acid (0.6 M). Then 20 ml of Scintiverse (Fisher Scienti®c, Fair Lawn, NJ) was added to each vial, and 14C ac-tivity was measured on a liquid scintillation counter. All values were corrected by quench curve and back-ground counts were subtracted. The 14C speci®c ac-tivity was calculated by dividing the14C activity by the total biomass of soil nematodes.

2.4. Statistical analysis

Unless otherwise stated, all measurements were made on samples from 0±5.0 cm depth, and all data

were expressed on a dry mass basis. Statistical analyses for all data were performed using SAS software (SAS Institute, 1985). Three-way ANOVA was carried out for decomposition time, soil depth and tillage. Com-parison among means was carried out using Tukey's test. Signi®cance levels were set atP<0:05:

3. Results

3.1. Total nematode numbers

In the laboratory study, total nematode numbers increased rapidly and peaked 1 month after residue ap-plication in both the 0±2.5 and 2.5±5.0 cm layers in the CT treatment, averaging 152 and 191 individuals gÿ1 _{soil over the entire experiment, respectively}

(Fig. 1A and B). In the NT, total nematodes increased signi®cantly in the 0±2.5 cm layer and only marginally in the 2.5±5.0 cm layer, averaging 140 and 74 individ-uals gÿ1 _{soil, respectively (Fig. 1A and B). At the}

beginning, soil nematode numbers were higher in NT than in CT, but the pattern was reversed 2 weeks after residue application.

In the ®eld study, soil nematode numbers did not show signi®cant changes under either tillage regime, but increased signi®cantly 40 days after residue appli-cation in the 0±2.5 cm layer and 67 days after the ex-periment began in the 2.5±5.0 cm layer in CT (Fig. 1C and D). Total nematode numbers were signi®cantly higher under NT than under CT in the ®eld.

In general, total soil nematode numbers increased more rapidly and became much more abundant under the laboratory conditions than those in the ®eld (Fig. 1A±D). The mean numbers from the laboratory study were 20±30 and 5±6 times more in the CT and NT than from the ®eld study, respectively.

3.2. Microbivorous nematodes and FN-to-BN ratio

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and the tillage eect was not as great as in the lab-oratory during this short period (Fig. 3C and D).

The ratio of fungivorous-to-bacterivorous nema-todes (FN-to-BN) was not signi®cantly dierent in CT and NT at the beginning of the residue application (Tables 1 and 2). The ratio was very low under both

tillage regimes for both laboratory and ®eld studies. However, it increased signi®cantly with time at the later stages of residue decomposition under CT for both laboratory and ®eld studies. In NT, however, no signi®cant change was observed except at the end of the experiment when there was an increase in the 2.5± Fig. 1. Dynamics of total nematodes after residue application: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C) 0±2.5 cm layer in ®eld, (D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are dierent from those of (C) and (D).

Table 1

Temporal changes of ratio of FN-to-BN after residue application in laboratory

Tillage Depth (cm) Time after residue application (days)

1 8 17 32 40

CT 0±2.5 0.1420.02b _0.17₂_0.03 _0.80a₂_0.19 _1.44a₂_0.12 _2.10a₂_0.25

2.5±5.0 0.1020.01 0.2520.05 0.49a20.04 1.13a20.02 1.3520.99

NT 0±2.5 0.1620.03 0.03a20.00 0.2220.05 0.2220.04 0.1420.05

2.5±5.0 0.1220.03 0.0220.00 0.2120.08 0.2620.08 0.43a20.09

a

Signi®cant atP<0:05. b

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Fig. 2. Changes of bacterivorous nematodes after residue application: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C) 0±2.5 cm layer in ®eld, (D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are dierent from those of (C) and (D).

Table 2

Temporal changes of ratio of FN-to-BN after residue application in ®eld

Tillage Depth (cm) Time after residue application (days)

1 13 32 40 49 67

CT 0±2.5 0.2220.06b _0.11₂_0.03 _0.22₂_0.04 _0.37₂_0.09 _1.18a₂_0.41 _1.00a₂_0.47

2.5±5.0 0.1620.05 0.1221.00 0.2420.03 0.2020.01 0.77a20.19 0.90a20.22

NT 0±2.5 0.1720.03 0.1020.02 0.0820.01 0.1520.02 0.64a20.13 0.34a20.06

2.5±5.0 0.2520.05 0.2220.13 0.1820.02 0.1020.00 0.2420.13 0.0920.07

a

Signi®cant atP<0:05. b

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5.0 cm layer in the laboratory study and in the 0±2.5 cm layer in the ®eld study (Tables 1 and 2).

3.3. Other trophic groups

In the laboratory study (Table 3), phytophages showed a signi®cant increase on days 32 and 40 in the 0±2.5 cm layer, and on days 17 and 32 in the 2.5±5.0 cm layer in CT, and increased signi®cantly on the days 8 and 40 in the 0±2.5 cm layer in NT. Predators showed a signi®cant increase on day 17 in the 2.5±5.0 cm layer and numbers declined thereafter, however, they did not increase signi®cantly until the end of the experiment in the 0±2.5 cm layer in CT. Predators also increased signi®cantly on days 8 and 32 after residue

application in the 0±2.5 cm layer in NT, but remained unchanged in the 2.5±5.0 cm layer throughout the ex-periment. Overall, predators accounted for only a small portion (less than 2%) of the total soil nema-todes. Omnivore numbers showed a trend of increasing with time after residue application in both layers and under both tillage regimes; however, the dierences were not statistically signi®cant.

In the ®eld study (Table 4), phytophage numbers remained unchanged in all cases. There were more phytophages under NT than under CT, but no depth eect was observed in either tillage treatment. Predator numbers did not show any changes in CT, but showed a signi®cant increase at the end of the experiment at both depths in NT. Omnivore numbers remained

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unchanged at both depths in the CT, but there was an increase on days 32 and 40 in the 0±2.5 cm layer and day 40 in the 2.5±5.0 cm layer in NT. Overall, phyto-phages, predators and omnivores were more abundant under NT than under CT in the ®eld study.

3.4.14C speci®c activity of soil nematodes

In the laboratory study, 14C speci®c activity of soil nematodes was the highest on day 8 of the experiment

and declined thereafter in both the 0±2.5 and 2.5±5.0 cm layers under CT; and it increased gradually but did not peak until 1 month later under NT. The 14C speci®c activity of soil nematodes was signi®cantly higher under CT than under NT. No signi®cant dier-ence of 14C speci®c activity of soil nematodes was found between the two layers under CT, but it was sig-ni®cantly higher in the 0±2.5 cm layer than in the 2.5± 5.0 cm layer under NT (Fig. 4A and B).

In the ®eld study, the 14C speci®c activity of soil nematodes was signi®cantly higher under CT than under NT. The 14C speci®c activity peaked on day 13 of the experiment and declined thereafter in both the 0±2.5 and 2.5±5.0 cm layers under CT; and it was hardly detectable until 1 month after residue appli-cation under NT. There was no signi®cant dierence of 14C speci®c activity of soil nematodes between the two depths under CT and NT (Fig. 4C and D).

4. Discussion

4.1. Early response of bacterivorous nematodes

In our study, bacterivorous nematodes responded much earlier and faster to residue application than fungivorous nematodes, while predators and omni-vores did not show any response until towards the end of the experiment. Our results were generally in agree-ment with other studies (Freckman, 1988; Ettema and Bongers, 1993; Griths et al., 1993; Bouwman and Zwart, 1994). In a plowed system in Sweden, Sohlenius and Bostrom (1984) found bacterivorous nematodes to be most abundant in buried plant residue during the early, rapid phase of decomposition, but fungivorous nematodes were most abundant later as decomposition slowed. Griths et al. (1993) found a large increase in the numbers of populations of microbivorous nema-todes during the decomposition of barley roots, with bacterivores initially dominant and then fungivores becoming dominant. Bouwman and Zwart (1994) observed that the Rhabditidae (bacterivores), in par-ticular, bloomed during the early stages of decompo-sition, whereas Cephalobidae (bacterivores) and Aphelenchoididae (fungivores), and ®nally, Tylenchi-dae (phytophages, in part, feed on fungi) became nu-merous at the later stages of decomposition of organic matter. Freckman (1988) concluded that the pattern of succession of bacterivores followed by fungivores is a common feature of organic matter decomposition, mir-roring microbial succession.

In contrast, our ®ndings challenged the explanation of Griths et al. (1993) for the fast increase of nema-tode populations. Griths et al. (1993) pointed out that the initial increase in nematode numbers was likely to result from preferential migration of nema-Table 3

Dynamics of other trophic groups of soil nematodes after residue ad-dition in laboratory (individual gÿ1_soil)

Tillage Group Depth (cm) Time after residue application (days)

1 8 17 32 40

CT Pa 0±2.5 7.3b 13.8 21.1 34.1c 36.1c

2.5±5.0 11.6 23.1 87.2c _98.1c _46.6

Pr 0±2.5 0.17 0.42 0.01 1.10 4.02c

2.5±5.0 0.01 0.01 11.25c _4.37 _1.19

Om 0±2.5 3.4 1.1 4.9 4.3 7.3

2.5±5.0 4.4 2.3 9.3 10.7 15.1

NT P 0±2.5 6.0 66.9c _51.8 _30.5 _80.7c

2.5±5.0 18.7 50.7 28.0 32.6 39.8

Pr 0±2.5 0.01 2.57c _1.05 _4.75c _0.69

2.5±5.0 0.01 0.83 1.73 0.12 0.63

Om 0±2.5 2.3 3.0 3.0 4.0 7.2

2.5±5.0 2.8 2.7 1.4 2.7 3.5

a

P, Om and Pr refer to phytophages, omnivores and predators, re-spectively.

b

Means of three replicates. Standard errors are not reported here due to limited space.

c

Signi®cant atP<0:05.

Table 4

Dynamics of other trophic groups of soil nematodes after residue ad-dition in ®eld (individual gÿ1_soil)

Tillage Group Depth (cm) Time after residue application (days)

1 13 32 40 49 67

CT Pa 0±2.5 3.7b 1.0 0.8 3.6 2.3 4.6

2.5±5.0 4.3 0.9 0.9 2.4 3.0 4.8 Pr 0±2.5 0.14 0.10 0.12 0.07 0.16 0.07

2.5±5.0 0.27 0.09 0.01 0.05 0.11 0.03

Om 0±2.5 1.6 0.6 0.7 2.6 1.4 2.0

P, Om and Pr refer to phytophages, omnivores and predators, re-spectively.

b

Means of three replicates. Standard errors are not reported here due to limited space.

c

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todes, and not from reproduction because nematodes could not reproduce rapidly enough to account for the increase. Nevertheless, rapid reproduction of soil nematodes was the only pathway to result in the increase of nematode populations in our laboratory ex-periment. Since soil cores were maintained in Mason jars in a growth chamber, no migration of nematodes was possible. This was more convincing in the CT treatment because the nematode numbers increased rapidly in both surface and deep soil layers. Anderson et al. (1981) examined the life cycles of two soil nema-todes in a laboratory microcosm study and found Mesodiplogaster lheritieri had a very fast generation time, 4 days, but the other speciesAcrobeloidessp. had a slower generation time of 11 days. It is apparent that

generation times of soil nematodes are species speci®c (Anderson and Coleman, 1981). Species composition might be dierent in our study as compared to the study of Griths et al. (1993) because corn residue rather than barley roots was used in our study. The quality of detritus in¯uences the type and growth of micro¯ora, and subsequently, their grazers (Freckman, 1988).

4.2. FN-to-BN ratio

Twinn (1974) reviewed the ratio of FN-to-BN for several sites, noting it is an indication of the import-ance of the two groups in the decomposition pathway. Bongers and Bongers (1998) also noted that changes in

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the relative abundance of bacterivores or fungivores mirror changes in the decomposition route. A low FN-to-BN ratio re¯ects the dominance of bacterivorous nematodes and may indicate abundant bacterial popu-lations (Freckman, 1988) and vice versa. Yeates et al. (1993) addressed the same issue by using the reverse ratio, BN-to-FN. They proposed that additional infor-mation on food sources for nematodes within the decomposer food web could be obtained by using the ratio of bacterivores-to-fungivores. This ratio is based solely on the ratio of numerical abundance and takes no account of body size, nematode activity or the graz-ing eects of the nematodes. A signi®cantly higher ratio implies a greater availability of food for bacteri-vores (Yeates et al., 1993).

In our experiments, the FN-to-BN ratio increased with time after residue application in the CT treatment and did not increase in the NT treatment until the end of the experiment. However, FN-to-BN ratios were not signi®cantly dierent at the beginning of the exper-iment between the CT and NT treatments, and were very low under both tillage regimes. In general, the ab-solute numbers of both bacterivores and fungivores were higher in NT than in CT. Our results indicated that bacterivorous nematodes had a greater in¯uence than fungivorous nematodes under both CT and NT treatments at the beginning of the experiment. Never-theless, the importance of fungivorous nematodes increased with time in the progress of residue de-composition under both tillage regimes, so the fungal pathway might be more important in CT than in NT shortly after residue application. Though fungivores did not show much increase in NT in our studies, their importance in NT might increase later as residue de-composition proceeds. To some extent, our results were in agreement with Brussaard et al. (1990) and Stinner and Crossley's (1982) studies; however, these were not consistent with Parmelee and Alston (1986) and the ®ndings of Beare et al. (1992). The timing of the sampling and the design of the experiment might be responsible for these consistencies and discrepan-cies.

4.3. Tillage and environmental eects

In agroecosystems, disturbance can have a strong in-¯uence on soil microbial populations and, sub-sequently, nematode communities. Coleman et al. (1983) concluded that intensively managed grasslands appear to correspond to the `fast cycle' dominated by labile substrates and bacteria, while less productive, organically fertilized grasslands relate to the `slow cycle' dominated by more resistant substrates and fungi. Sohlenius and Bostrom (1984) found that recov-ery of nematode populations from a moderate disturb-ance, such as tillage or fertilization, resulted in an

increase in the dominance of opportunistic bacteri-vores. Dmowska and Kozlowska (1988) also pointed out that plowing stimulates mineralization and results in increase of nematode numbers and dominance of the opportunistic taxa.

In our studies, signi®cant tillage eects were observed in most cases. All trophic groups were signi®-cantly higher in NT than in CT in the ®eld study, and at the beginning of the laboratory study. Microbivor-ous nematodes (bacterivores and fungivores) responded to residue addition much earlier and their numbers increased much faster under CT than under NT. 14C speci®c activity was signi®cantly higher under CT than under NT. Our results agreed with that dis-turbance (e.g. plowing) can have a strong in¯uence on soil nematodes (Dmowska and Kozlowska, 1988; Ettema and Bongers, 1993; Freckman and Ettema, 1993). However, there were two sources of soil disturb-ances in our experiment treatments: application of crop residue (resource enrichment) and plowing (physi-cal disturbance). The disturbances in CT were greater than in NT treatment since crop residue was well mixed into soil under CT by plowing, while crop resi-due was only applied on soil surface without plowing under NT. How to dierentiate from each other the eects of the two sources of disturbances on soil nema-todes under dierent tillage regimes? We suggest that another set of treatment with or without plowing in the absence of crop residue addition be carried out simultaneously.

There was a signi®cant depth eect on the response of total nematodes, bacterivores, fungivores and 14C speci®c activity under NT, but not under the CT treat-ment in both the laboratory and ®eld studies. Further-more, in the laboratory study, fungivore numbers started increasing 2 weeks after residue application in CT but did not increase throughout the entire period in NT. In the ®eld study, fungivore numbers started increasing 1 month after residue application at both layers in CT and at the surface layer, but not in the deep layer of the NT treatment. Vertical distribution of trophic groups of soil nematodes re¯ected the distri-bution and abundance of their food sources. Faster residue decomposition rate and more uniformly dis-tributed organic matter by plowing eliminated the depth eect in CT, whereas the strati®cation of organic matter on the soil surface resulted in the dierent re-sponse of nematodes at dierent soil depths in NT.

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moder-ate soil temperature ¯uctuations. In contrast, the rela-tively bare soil in CT resulting from residue incorpor-ation has high potential evaporative water losses and large daily temperature ¯uctuations (Holland and Coleman, 1987). Favorable soil temperature and moisture enable soil microorganisms and soil animals to reproduce faster in the laboratory than in the ®eld where soil temperature and moisture ¯uctuates.

Since the 14C speci®c activity of soil nematodes and the ratio of 14C bound in nematode biomass to total

14

C decayed in the experiment (reported elsewhere) were signi®cantly higher under CT than under NT in both the laboratory and ®eld studies, it is logical to infer that soil nematodes use carbon more eciently under CT than under NT. Although the 14C speci®c activity of soil nematodes was higher in the ®eld than in the laboratory study, the ratio of 14C bound in nematode biomass to total 14C decayed in the exper-iment (reported elsewhere) was lower in the ®eld than in the laboratory. Where do soil nematodes use carbon more eciently, in the ®eld or in the laboratory? The answer remains uncertain.

The 14C speci®c activities of soil nematodes in our study were lower compared with Yeates et al.'s (1998) study. Is it because of dierent substrates used in two studies (crop residue in our study and root exudates in theirs) or because of dierent groups of nematodes selected for measurements (the entire nematode com-munity considered in our study and only one sedentary species selected in their study)? The question cannot be answered without substantial evidence of C-to-N ratio, lignin contents of the substrates and other character-istics of the nematode community.

Acknowledgements

We thank Keith W. Kisselle, Carol J. Garrett, Betty Weise, Paula Marcinek, Patricia Huback, Kathy Sas-ser, Brent Andrews and Sherry Farly for their ®eld and laboratory assistance. Special thanks from the senior author to Dr Christien H. Ettema for her tech-nical instruction and insightful discussion. Dr Christien H. Ettema, Stephanie Madson, two anonymous reviewers and the Editor-in-Chief (Dr John Waid) of the journal greatly improved the manuscript. This study was supported by a grant from the National Science Foundation to the Institute of Ecology, Uni-versity of Georgia.

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