The Spatial Distribution of Nematode Trophic Groups Across a Cultivated Ecosystem

In order to better understand the spatial distributions of soil trophic groups and the potential significance of these distributions to ecosystem functioning we initiated a study to describe the within—site variability of nematode feeding groups in a row—crop ecosystem. Soil cores were removed from a 48—ha corn (Zea mays) field in the U.S. Midwest prior to spring planting, and nematodes were identified by phenotypic criteria to four groups: bacterivores, fungivores, omnivores/predators, and plant parasites. Within—site variability was high for all groups; population counts spanned two orders of magnitude, with coefficients of variation ranging from 40—130% (n = 115—138 soil samples). Probability distributions were strongly lognormal. Geostatistical analysis showed that a major part of this variability was spatially dependent; variograms suggest that 70—99% of sample population variance was related to spatial autocorrelation over our geographic range of 6—80 m, except for the parasitic group, for which we detected no autocorrelation to 1200m. Maps of nonparasitic feeding groups across the field showed large multi—hectare areas of low to moderate population densities, with sub—hectare clusters of high—density populations towards one end of the site. Individual feeding groups were only weakly correlated with one another across the field (Kendall's t < 0.363, P < 0.001). Edaphic factors (bulk density, texture, pH, C availability, N availability) could collectively explain <30% of the variability in the nonparasitic groups across the area sampled. Results suggest that important soil food web components are strongly patterned at sub—hectare scales in this site. That this patterning is maintained in an ecosystem subjected to the homogenizing influences of annual soil tillage and a monoculture plant population is remarkable, and suggests that such patterning may be even more common in less—disturbed sites. Inclusion of these patterns in studies of ecosystem processes and soil community dynamics may significantly improve soil trophic models and our understanding of the relationship between soil populations and ecosystem function.