MEASURES OF NEMATODE COMMUNITY STRUCTURE AND SOURCES OF VARIABILITY AMONG AND WITHIN AGRICULTURAL FIELDS

Whole nematode communities, extracted from soil samples taken from agr icultural fields, were enumerated by taxonomic family and trophic grou p (i.e., bacterivores, fungivores, omnivores, plant-parasites, and pre dators) to evaluate nematode community structure as an indicator for m onitoring ecological condition of soil. No differences were found in m ixing treatments or methods of packing or shipping samples. However, e xtraction using Cobb's sifting and gravity method, followed by sucrose centrifugation, gave greater recovery of free-living nematodes than e lutriation followed by sucrose centrifugation. Population means and va riance of the sampled area were similar when sampled using different s trategies for collecting soil samples within fields, including several patterns, directions and repetitions of transects. Components of vari ation associated with ratios among the five trophic groups of nematode s and selected indices of community structure were quantified as varia tion among regions, among counties, among agricultural fields (2-ha ar ea), among transects within agricultural fields, and within composite soil samples. The variance component for 'within composite soil sample s' was relatively large compared to the other components of variance. Variation within composite soil samples was less for maturity indices (based on life-history strategy characteristics), ratio of bacterivore s to plant-parasites, sum of bacterivores and fungivores, populations of plant-parasites, and populations of bacterivores than for trophic d iversity indices, populations of fungivores, populations of omnivores, populations of predators, or the ratio of fungivores to bacterivores. With a single composite sample per field, the ability to differentiat e ecological condition of soils among fields within a region improved if the variance among and within fields exceeded the variance within c omposite samples. Given the variance components, power curves indicate d that detection of a 10% change (with 0.8 power) in the ecological co ndition of soils within a region between two time periods would requir e sampling a minimum of 25 and 50 fields with one composite soil sampl e analyzed per field for the maturity and trophic diversity index, res pectively. More than 100 fields per region would be required to detect temporal change in populations of individual trophic groups. Biplots of maturity indices, but not of trophic diversity or populations of in dividual trophic groups, identified clear differences among fields. Th us, maturity indices, which differentiated among sampling sites better and more efficiently than trophic diversity indices or measures based on populations of individual trophic groups, may be appropriate for u se in a regional and/or national monitoring program.