Effects of altered soil-water availability on a
tallgrass prairie nematode community
T.C. Todd
*, J.M. Blair, G.A. Milliken
Department of Plant Pathology, Division of Biology and Department of Statistics, Kansas State University, Manhattan, KS, USA
Received 28 September 1998; received in revised form 17 March 1999; accepted 17 March 1999
Abstract
Climate change predictions for the Great Plains region of North America include reduced growing season precipitation. The consequence of this prediction for soil fauna and belowground processes was investigated at two spatial scales by integrating experimental manipulation of soil moisture levels with natural variation in soil-water availability. Experiments consisted of (1) reciprocal core transplants across a regional precipitation gradient and (2) supplemental irrigation applied across a local topographic gradient. This report examines functional-level responses by the tallgrass prairie nematode community to differences in soil moisture levels over a four-year period. Effects on nematode community structure were complex and dependent upon nematode trophic habit and depth in the soil pro®le. The strongest and most consistent responses to changes in soil-water availability were displayed by herbivorous taxa, with 71% higher densities observed under wetter soil conditions across experiments and years. Responses of microbial-feeding nematodes were more variable, with lower densities observed, in some cases, in the presence of experimentally-increased soil moisture levels. Effects of regional differences in soil-water availability on the nematode community were uniformly restricted to depths >20 cm. Community responses to short-term changes in soil moisture were not consistent with patterns in community structure developed under different natural moisture regimes, suggesting divergent short-term and long-term responses of belowground biota and processes to changes in soil-water availability.#1999 Elsevier Science B.V. All rights reserved.
Keywords:Climate change; Community structure; Nematode; Soil moisture; Tallgrass prairie
1. Introduction
The Great Plains region of North America is domi-nated by grassland ecosystems whose distributions are determined primarily by an east±west precipitation gradient (Weaver, 1954; Risser et al., 1981). Grassland
types range from the xeric shortgrass prairie at the western edge of the region to the mesic tallgrass prairie at the eastern edge. Primary productivity across this gradient is also related strongly to precipitation (Sala et al., 1988). Within grassland types and parti-cularly for frequently-burned tallgrass prairie, often soil moisture is the best predictor of aboveground net primary productivity because it re¯ects longer-term patterns of precipitation and water use (Knapp et al., 1998).
*Corresponding author. Tel.: 1350; fax: +785-532-5692; e-mail: nema@plantpath.ksu.edu
Recent models of global climate change include predictions of decreased growing season precipitation for the Great Plains region (Karl et al., 1991). The generic effects of reduced precipitation on above-ground productivity in North American grasslands can be surmised based on spatial (east±west gradient) and temporal (drought cycles) patterns in these two variables but much less is known about the relation-ships between soil moisture and belowground pro-cesses for these same ecosystems. Pulses of root growth, decomposition, and biotic activity have been observed following precipitation events (Hayes and Seastedt, 1987; Elliot et al., 1988) but long-term belowground responses to altered precipitation pat-terns have not been investigated. Given that a large proportion of primary production in temperate grass-lands occurs belowground (Coleman et al., 1976; Risser et al., 1981) and that processes such as nutrient cycling are strongly regulated by soil fauna (Hunt et al., 1987; Elliot et al., 1988; Seastedt et al., 1988), a better understanding of the interaction of soil moisture regimes, ecosystem processes, and belowground food webs is essential to any evaluation of the long-term consequences of climate change for Great Plains grasslands.
Nematodes represent a major component of soil food webs in grassland ecosystems, interacting with other soil biota through multiple trophic pathways (Elliot et al., 1988; Ransom et al., 1998). Their in¯uence on ecosystem processes in grasslands has been well documented and includes effects on primary production through herbivory (Smolik, 1977; Stanton et al., 1981; Ingham and Detling, 1990) and on nutrient cycling through grazing of microbial decom-posers (Ingham et al., 1985, 1986a, b; Hunt et al., 1987). The trophic structure of the nematode commu-nity has been shown to be a reliable index of soil food web structure, particularly in relation to decomposi-tion pathways (Neher and Campbell, 1994; Todd, 1996). Based on these characteristics, an understand-ing of nematode community responses (as well as those of other soil fauna) to altered precipitation patterns appears to be a prerequisite for predicting ecosystem-level responses to climate change.
The objective of the present study was to assess nematode community responses, including changes in trophic structure, temporal dynamics, and vertical distribution, to experimentally-altered soil-water
availability at two spatial scales: (1) a regional scale, using reciprocal core transplants across a naturally-occurring precipitation gradient; and (2) a local scale, using supplemental irrigation across a topographic gradient encompassing natural differences in soil-water availability. Both experiments were designed to assess the long-term effects of differences in soil-water availability on soil fauna (using existing natural gradients), as well as determine the short-term responses of the faunal community to experimental manipulation of soil moisture. Responses of soil microarthropods have been published separately (O'Lear and Blair, 1999).
2. Materials and methods
2.1. Reciprocal core transplant experiment
The study was conducted at two sites across an east±west precipitation gradient. The more mesic site was located at the Konza Prairie Research Natural Area (KPRNA), a 3487 ha tallgrass prairie located 12 km south of Manhattan, Kansas (398050
Seventy intact soil cores (25 cm diameter70 cm deep) encased in open-ended PVC cylinders were extracted from each site during the autumn of 1993 using a hydraulic soil coring machine (Swallow et al., 1987). Cores were collected from areas within each site that were dominated byA.gerardii. Half of the cores were replaced in their original holes and the remaining half were transplanted into holes at the reciprocal site. The experimental design at each location consisted of 35 sets of paired cores (one from each site) arranged in a randomized complete block design. Five pairs of cores were collected from each site for destructive sampling in May and October/November of each year (1994± 1996) and sectioned into 0±10, 10±20, 20±40, and 40±60 cm depth increments. The remaining ®ve pairs of cores were reserved for future sampling to assess longer-term trends.
Nematodes were extracted from 100 cm3 subsam-ples of mixed soil from each depth section following uniform mixing using a modi®ed Christie±Perry tech-nique (Christie and Perry, 1951). Counts were adjusted based on an average extraction ef®ciency of30% for the dominant taxa present (Seastedt et al., 1987). Nematodes were identi®ed to family or genus level and assigned to the following trophic groups based on Yeates et al. (1993) and Todd (1996): (1) herbivore (root-feeding; Helicotylenchus and Paratylenchinae comprised 68% of this group across dates and loca-tions); (2) fungivore (hyphal-feeding; 67% Tylench-idae); (3) microbivore (bacterial- and unicellular eucaryote-feeding; 52% Cephalobidae); (4) omni-vore/predator (primarily invertebrate feeding; 89% of this group consisted of large species in the Dory-laimida). A complete list of the dominant nematode taxa of the KPRNA tallgrass prairie site and their trophic groupings can be found in Ransom et al. (1998).
The data were analyzed as a strip±strip plot design with core origin, depth, and sampling date as strip factors nested within location. The analysis used a strip-plot model (Milliken and Johnson, 1992) and the GLM and Mixed procedures in SAS (Statistical Ana-lysis Systems Institute Inc., 1989; Littell et al., 1996). Nematode densities were log10-transformed to reduce heterogeneity of variances for analysis of variance and mean comparisons. Untransformed means are pre-sented in tables and ®gures.
2.2. Irrigation transect experiment
The study was conducted on an annually burned KPRNA site with the same general characteristics as described for the reciprocal core transplant study. The experiment utilized two existing replicate irrigation pipelines (140 m in length), which traversed a topo-graphic gradient of soil-water availability ranging from drier uplands to wetter lowlands (Knapp et al., 1994). The upland soil in this gradient was a Clime± Sogn complex (®ne, mixed mesic Udic Haplustoll) and the lowland soil was an Irwin silty clay loam (®ne mixed mesic Pachic Argiustoll). Both soils were texturally similar (21±22% sand, 41±44% silt, 34± 38% clay) and both sites contained vegetation char-acteristic of the tallgrass prairie, as described above. Supplemental irrigation was applied during May through September of each year to offset growing season moisture de®cits. Timing and amounts of irrigation supplements were based on measured rain-fall amounts and estimated evapotransporation (Knapp et al., 1994). Growing season precipitation and supplemental irrigation amounts, respectively, were 735 and 357 mm for 1995, 561 and 234 mm for 1996, and 400 and 313 mm for 1997.
Sixteen 3 m diameter plots were established for sampling, with two replicate irrigated and two repli-cate nonirrigated control plots lorepli-cated on each of two topographic sites (upland and lowland) per pipeline. This resulted in four plots for each site-treatment combination. Irrigated plots within a topographic site were located on opposite sides of each of the two irrigation pipelines (2±3 m from the pipeline); control plots were similarly located along each of two parallel, non-irrigated control transects (O'Lear and Blair, 1999).
3. Results
3.1. Reciprocal core transplant experiment
Mean annual precipitation at FHAES was 42% less than that at KPRNA (616 vs. 1053 mm) during the present study. For all years of the study, differences in
annual precipitation between locations (32±56%) were consistently larger than the long-term average difference of 30%. Seasonal patterns of precipitation for both locations have been reported in O'Lear and Blair (1999).
Table 1 summarizes the variation in nematode den-sities associated with location (site where the cores
Table 1
Analyses of variance and main effect means for nematode trophic group densities from the reciprocal core transplant experiment, 1994±1996
Source of variation df Mean squares
herbivores fungivores microbivores omnivore/predators
Year 2 5.14 3.73 0.12 7.80
Month 1 1.08 0.77 35.04** 73.61**
Monthyear 2 3.70 5.60* 21.78** 16.34*
Error (month year) 40 1.82 1.47 1.98 4.90
Location 1 12.19* 3.60 0.82 0.12
Error (loc) 8 1.78 1.70 3.12 3.43
Origin 1 145.38** 13.48* 71.50** 4.73
Originloc 1 0.15 8.07* 53.71** 17.70
Error (origin loc) 8 2.39 1.54 2.51 13.90
Depth 3 130.98** 16.53** 223.60** 171.01**
Depthloc 3 1.49 0.49 3.29 13.65*
Error (depth loc) 24 1.21 1.07 5.30 3.52
Depthorigin 3 139.38** 7.61** 33.25** 5.56
Depthoriginloc 3 5.34** 4.80** 9.47 6.54
Error (depth origin loc) 24 0.99 1.24 3.43 5.19
Main effect means (thousands per square meter)
Year(n160)
1994 670 596 287 89
1995 786 748 277 104
1996 766 629 301 70
Month(n240)
May 602 470 260 80
October 879 845 316 96
Location(n240)
FHAES 589 611 296 81
KPRNA 893 705 280 95
Origin(n240)
FHAES 953 662 325 82
KPRNA 529 654 251 94
Depth(n120)
0±10 cm 558 943 604 138
10±20 cm 723 536 228 75
20±40 cm 1041 703 214 77
40±60 cm 641 450 106 61
resided during the experiment) and origin (site where the cores originated), as well as sampling date and depth. Although core origin consistently accounted for a larger portion of the variation in nematode densities than did location, the implied importance of historic soil moisture conditions is misleading. Major differ-ences due to core origin were restricted exclusively to the lowest soil depth (see Fig. 1), resulting in large depthorigin interaction terms (Table 1). This effect appeared to be adequately explained by textural dif-ferences between the two soils at that depth (see Section 2). Abrupt decreases in nematode densities in KPRNA cores below the 40 cm depth (Figs. 1 and 2), were coincident with a notable increase in clay content. No such delineation in both soil texture and nematode densities was observed for FHAES cores.
This pattern was present at initiation of the experiment and remained constant throughout the sampling per-iod. When the 40±60 cm depth was excluded from the analyses of variance, no effects due to core origin were observed. Thus, no long-term effects of soil moisture differences were discernable in the nematode data.
In contrast to the absence of an identi®able effect of historic soil moisture, differences in soil moisture conditions between locations during the present study did in¯uence nematode densities. Herbivorous nema-tode densities were 52% higher (p0.03) at KPRNA than at FHAES across cores, depths, and years. Her-bivore, fungivore, and microbivore densities all exhib-ited three-way interactions (p0.06) among origin, location, and depth of soil cores, with differences between locations consistently restricted to the lower
Fig. 1. Densities of herbivorous nematodes in cores originating from (a) FHAES and (b) KPRNA at four depths and two locations averaged across month and year. An asterisk indicates a significant difference between locations within depth according to least-squares means of log10-transformed data (p0.05). Data are
means of 30 observations.
Fig. 2. Densities of microbivorous nematodes in cores originating from (a) FHAES and (b) KPRNA at four depths and two locations averaged across month and year. An asterisk indicates a significant difference between locations within depth according to least-squares means of log10-transformed data (p0.05). Data are
soil pro®les (Figs. 1 and 2). Herbivorous nematode densities at the lower soil depths tended to be higher in cores residing at KPRNA compared to those at FHAES, regardless of core origin (Fig. 1). In contrast, the effect of location on microbivorous nematodes, while still restricted to the lower soil depths, depended upon core origin. Microbivore densities in FHAES cores were lower (p0.05) when residing at KPRNA vs. FHAES, while those in KPRNA cores were higher (p0.05) at KPRNA vs. FHAES (Fig. 2). Fungivore densities displayed patterns similar to those observed for microbivores (data not shown). Locationyear interactions (p0.05; not included in Table 1) also occurred for several trophic groups. Fungivore and omnivore/predator densities were higher at KPRNA in 1995, the year with the greatest difference in preci-pitation between locations (O'Lear and Blair, 1999), but tended to be lower in other years. Lower micro-bivore densities were observed at KPRNA compared to FHAES in 1996.
As detailed above, a high clay content in the 40± 60 cm depth of the KPRNA cores restricted densities of all trophic groups, resulting in large origindepth interaction effects (Table 1). Above this depth, nema-tode densities were generally distributed similarly in the two soil pro®les, although vertical distribution of individual trophic groups varied considerably. Micro-bivores were the most strati®ed group, with >50% of the total population occurring at the 0±10 cm depth (Table 1, Fig. 2). Fungivore and omnivore/predator groups were also concentrated in the upper 10 cm of the soil pro®le but to a lesser extent than micro-bivores (Table 1). Hermicro-bivores were more evenly dis-tributed in the soil pro®le (above 40 cm) in cores originating from KPRNA, or tended to be concen-trated at lower soil depths in cores originating from FHAES (Table 1, Fig. 1).
Variability in the depth distribution of nematodes occurred across sampling dates, as indicated by large depthmonth and depthyear interactions (p< 0.01; not included in Table 1) for several groups. This variability did not, however, encompass large changes in the general patterns of vertical distribution already described. For example, microbivore densities were more strongly strati®ed on autumn than on spring sampling dates but densities remained highest at the 0±10 cm depth and lowest at the 40±60 cm depth in both cases.
3.2. Irrigation transect experiment
The moisture content of soil cores collected for nematode assay was 19% higher, on average, in irrigated plots across sampling dates but moisture differences varied with month and year (p< 0.001). Soil moisture was signi®cantly higher (p0.05) in irrigated vs. control plots on ®ve of eight dates (Fig. 3). Moisture content did not vary between sites (upland vs. lowland). Volumetric soil water content, determined by a time domain re¯ectometry (TDR) soil moisture monitoring system (Campbell Scienti®c), and daily precipitation amounts for these plots have been reported by O'Lear and Blair (1999) for 1995, the year with the greatest observed differences in soil moisture between irrigation treatments.
Herbivores were the only trophic group to display a moderately consistent response to irrigation across sampling dates, with a 90% increase (p0.07) in average densities in irrigated vs. control plots. Irriga-tiondate interactions (p0.08) were observed for herbivore, fungivore, and microbivore densities (Table 2). Herbivore densities were higher (p0.05) in irrigated plots on four of eight dates, with signi®cant responses observed in every year of the study (Fig. 4(a)). Microbivorous nematodes, in contrast, exhibited higher (p0.05) densities with irrigation only on the July 1995 sampling date when soil moisture differences between treatments were greatest (Fig. 4(b)). Densities of this trophic group
Fig. 3. Gravimetric soil moisture content of cores collected for nematode extraction from control and irrigated plots averaged across upland and lowland sites. An asterisk indicates a significant difference between irrigation treatment within date according to least-squares means of log10-transformed data (p0.05). Data are
tended to be comparable or lower in irrigated vs. control plots on most of the other sampling dates. Similarly, fungivore densities were higher (p0.05) in irrigated plots on the July, 1995 and June, 1996
sampling dates but no consistent responses were observed across the remaining dates (data not shown). Irrigation resulted in 15% higher omnivore/predator densities in upland soils but 14% lower densities in
Table 2
Analyses of variance and main effect means for nematode trophic group densities from the irrigation transect experiment, 1995±1997
Source of variation df Mean squares
herbivores fungivores microbivores omnivore/predators
Year 2 0.797** 0.276* 0.721** 1.667**
Month (year) 5 0.831** 1.288** 0.921** 0.258
Error (month year) 7 0.058 0.059 0.057 0.087
Site 1 0.004 0.055 0.105 0.085
Error (Site) 1 0.025 0.056 0.020 0.147
Irrigation 1 4.849 0.270 0.111 0.034
Error (irr) 1 0.067 0.018 0.004 0.463
Irrsite 1 0.106 0.139 0.017 0.299*
Error (irr site) 1 0.014 0.002 0.012 0.001
Siteyear 2 0.156 0.154* 0.072 0.036
Sitemonth (year) 5 0.073 0.112* 0.104 0.150
Error (site month year) 7 0.218 0.023 0.085 0.087
Irryear 2 0.586** 0.104 0.228 0.130
Irrmonth (year) 5 0.344* 0.230* 0.258 0.068
Error (irr month year) 7 0.052 0.032 0.078 0.115
Irrsiteyear 2 0.008 0.037 0.057 0.044
Irrsitemonth (year) 5 0.023 0.018 0.242 0.068
Residual error 7 0.082 0.055 0.118 0.045
Main effect means (thousands per square meter)
Year(n48)
1995 1541 2151 830 240
1996 1420 1888 822 207
1997 2214 2737 1231 438
Month(year) (n16)
July 95 1341 1987 621 216
October 95 1740 2315 1040 264
June 96 876 959 496 188
August 96 1087 1287 665 121
October 96 2297 3417 1304 313
June 97 1435 1271 717 408
August 97 2585 2427 1142 444
October 97 2622 4513 1832 463
Site(n64)
Upland 1843 2257 903 291
Lowland 1653 2287 1051 313
Irrigation(n64)
Control 1206 2256 961 309
Irrigated 2290 2289 993 295
lowland soils (signi®cant irrigationsite interaction, p0.04) across dates.
Topographic effects were rare and, when observed, varied across sampling dates. Fungivorous nematodes were the only trophic group displaying dissimilar population densities between upland and lowland sites, with differences dependent on both month and year (Table 2). This group tended to be more abundant in upland soils early in the growing season and more abundant in lowland soils late in the growing season (data not shown).
4. Discussion
The responses of individual nematode trophic groups to experimentally-altered soil moisture levels were surprisingly consistent across experiments, sea-sons, and years. The largest responses were observed with herbivorous taxa, where increased soil moisture due to core transplantation or irrigation resulted in increases of 52% and 90%, respectively, in nematodes densities across years. The magnitude of the response in the mesic tallgrass prairie irrigation experiment was unexpected given that growing-season precipitation
across years was within 10% of the long-term average. This contrasts with observations from the more xeric shortgrass prairie, where plant-feeding nematode responses to supplemental irrigation and nitrogen were highly variable and often neutral (Smolik and Dodd, 1983). A positive relationship between soil moisture and herbivorous nematode densities has been observed on wheatgrasses in the Utah desert (Grif®n et al., 1996). Of course, individual nematode species will exhibit preferences for speci®c soil moisture condi-tions (Schmitt and Norton, 1972) and functional group responses to changes in water availability are likely to vary both within and among ecosystems, depending on the dominant taxa present. Nevertheless, the short-term responses of total herbivore densities to increased (or decreased) soil moisture levels were consistent across a range of environments in the present study.
The remaining trophic groups displayed few sig-ni®cant responses to changes in soil moisture. Fungi-vore and microbiFungi-vore densities were higher in the presence of supplemental irrigation on the date with the lowest soil moisture content (and the greatest difference in soil moisture between treatments) but other responses to increased soil moisture were as likely to be negative as positive, particularly for the microbivores. In this case, the response is consistent with the inverse relationship between the C : N ratio of microbial biomass and water availability in tallgrass prairie, re¯ecting an increase in the activity of bacteria relative to fungi under drier conditions (Rice et al., 1998). Microbivore densities have been observed to increase concomitantly with increases in microbial N and bacterial biomass in tallgrass prairie soils (Todd, 1996). Reduced microbivore densities under wetter soil moisture regimes would, therefore, be consistent with expected changes in microbial communities.
In the present study, microbivore populations which developed under different soil moisture conditions displayed differential responses to altered soil-water availability, re¯ecting the importance of historic soil moisture conditions. Whether this results from site-related differences in nematode species composition or from belowground food webs or processes that have diverged under long-term differences in soil-water availability remains to be determined. Regardless of the mechanism, trends in nematode responses to soil-water availability do appear to differ among ecosys-tems. Our observations contrast with those from the
Fig. 4. Densities of (a) herbivores and (b) microbivores in control and irrigated plots averaged across upland and lowland sites. An asterisk indicates a significant difference between irrigation treatment within date according to least-squares means of log10
shortgrass prairie, for example, where fungivorous (Tylenchidae), microbivorous, and predaceous nema-todes were all more responsive to supplemental water and N than herbivorous nematodes (Smolik and Dodd, 1983). In a recent study of a perennial agricultural system, only omnivorous genera of the Dorylaimida responded (positively) to irrigation intensity (Pora-zinska et al., 1998). Such con¯icting results emphasize that nematode responses to changes in climatic vari-ables, such as water availability, should not be general-ized across ecosystem types. Functional group responses to changes in soil-water availability will re¯ect the preferences of the major constituent taxa present, as previously noted, but will also be deter-mined by resultant changes in belowground processes as mediated by historic conditions.
Seasonal patterns of herbivorous nematode densi-ties in tallgrass prairie are typically related to root phenology and biomass, especially during periods of drought (Rice et al., 1998). Large decreases in root production have been documented during drought years at KPRNA (Hayes and Seastedt, 1987) but, in the present study, root responses to altered soil moist-ure levels did not follow this simple trend. While cores originating from FHAES had lower belowground plant biomass than those originating from KPRNA as expected, there was a trend for reduced below-ground biomass at the more mesic KPRNA site com-pared to the more xeric FHAES site following core transplantation (Blair et al., in preparation). Addition-ally, estimates of root biomass were not signi®cantly affected by irrigation at KPRNA, although root bio-mass was greater in lowland vs. upland plots (Todd, unpublished data). Together, these observations sug-gest that herbivorous nematode responses to changes in soil moisture result from abiotic effects and are independent of root responses. This hypothesis is improbable, however, based on the comparative lack of, or inverse, responses of the remaining trophic groups to increased soil moisture levels and given compelling arguments to the contrary (Grif®n, 1984; Todd, 1996; Rice et al., 1998). It is more likely that the dynamic nature of root production and the rapid turn-over of roots in prairie soil confounded the relation-ship between precipitation patterns and root dynamics in the present study. Pulses in new root production, as measured by root windows, are ephemeral, with much higher rates of turnover than can be measured for root
biomass (Hayes and Seastedt, 1987). Alternatively, the herbivore response may re¯ect the increased N avail-ability that accompanied wetter conditions in the reciprocal core experiment (Blair et al., in prepara-tion). The tallgrass prairie is characterized by N limitation and herbivorous nematode densities in this ecosystem respond positively to the increased nutrient status of roots following N fertilization (Todd, 1996). The experiments in the present study were designed to encompass both short- and long-term effects of soil-water availability. Transplanted cores in the reciprocal core transplant experiment permitted investigation of the relationship between changes in soil-water avail-ability and short-term responses by the nematode community, while comparison of soil cores from their respective sites of origin provided insights about nematode communities derived under different annual precipitation regimes. Similarly for the irrigation transect experiment, short- and long-term nematode responses to changes in soil-water availability could be deduced, respectively, through comparison of irri-gation treatments and topographical sites (soil moist-ure and net primary production are typically greater in lowlands than in uplands; Knapp et al., 1993; O'Lear and Blair, 1999). In both instances, short-term responses were inconsistent with long-term differ-ences. For example, short-term increases in herbivor-ous nematode densities in response to increased soil moisture levels were documented in both experiments but herbivore densities did not differ between soil cores at their site of origin (with the exception of the 40±60 cm depth, where differences were related to soil texture, not location), nor did they differ between upland and lowland sites, despite greater root biomass for the lowlands (Todd, unpublished data). Thus, it appears that the observed nematode responses may not be indicative of more complex long-term responses to changes in water availability. Resolution of this issue awaits the longer-term measurements planned for the present study.
is important to further note that all of the signi®cant differences in nematode densities between locations occurred at depths below 20 cm, regardless of soil origin. These observations emphasize that grassland nematode communities and their responses to climate change may not always be accurately represented with the standard 20 cm sampling depth.
5. Conclusions
Our data indicate that altered soil-water availability related to potential changes in climate in the Great Plains region will result in complex changes in the structure of soil food webs in the grasslands of the region. Tallgrass prairie nematode community responses to experimentally-altered soil moisture were dependent upon trophic habit, with plant-feeding taxa responding rapidly and favorably to increased moisture levels, while microbial-feeding taxa were as likely to be negatively as positively impacted. These variable responses appear to be the opposite of expected responses from nematode communities in the more xeric grasslands of the region (Smolik and Dodd, 1983). Further complicating any predictions of grassland ecosystem responses to climate change, O'Lear and Blair (1999) reported that soil microar-thropod communities in tallgrass prairie were nega-tively impacted by increased soil water content, suggesting that responses among soil invertebrate groups in this ecosystem are likely to be highly variable. Finally, the data imply that short-term responses by soil biotic communities may not suf®-ciently predict the long-term consequences of climate change, emphasizing the necessity of assessing such responses over relatively long time scales.
Acknowledgements
The authors thank T. Oakley and the staff at Konza Prairie Research Natural Area and the KSU Agricul-tural Research Center at Hays for ®eld and laboratory assistance and plot maintenance. Research was funded by the US Department of Energy's National Institute for Global Environmental Change (DOE/NIGEC) through the NIGEC Great Plains Regional Center at the University of Nebraska, Lincoln. Financial support
does not constitute DOE endorsement of the views expressed in this article. Contribution no. 99-119-J from the Kansas Agricultural Experiment Station, Manhattan.
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