Effects of soybean-wheat cropping and tillage regimes on nematode population and community dynamics

Increased utilization of conservation tillage and cropping pro-grams has necessitated a better understanding of nematodes in these programs. This study was undertaken to better characterize the nematode community structure in various soybean-wheat cropping and tillage regimes, and to assess the effects of these regimes on nematode com-munity and population dynamics. Sample plots were located at Spring Hill and Milan, Tennessee, and were arranged as randomized block designs with five treatments at each location. Soil samples were taken in May, July, and October, 1982. Nematodes were extracted from 100cm³ aliquants by means of a sugar flotation-centrifugation technique, and identified to species or species-level taxa. Similarities among overall nematode communi-ties, as well as among trophic group communities, for each treatment-replicate sample at each site were calculated by means of a community ordination technique. Species dominance and species diversity were calculated for each sample. Densities of species occurring in 50 percent or more of the samples at Spring Hill and Milan were tested for significant differences. All means were separated by the Waller- Duncan K-ratio t-test. One hundred nematode species were found in the experimental plots with 74 and 69 species occurring at Spring Hill and Milan, respect-ively. There were no significant differences in overall diversity and dominance, or in these values for any of the trophic groups; thus changes in cropping and tillage regimes for one season have lit-tle effect on nematode communities. This also indicates that the Dorylaimida may not be as reliable as was previously thought as in-dicators of ecosystem stability. Densities of Heterodera glycines Ichinohe infective juveniles were significantly higher in single-cropped, conventionally tilled soybeans than in any of the double-cropped treatments on the July sample date. Use of winter wheat cover crop as a possible control for H. glycines needs further inves-tigation. Community ordination of trophic groups yielded useful information only for the plant parasites. Plant-parasitic communities in conven-tionally tilled single-cropped soybeans tended to aggregate when opposed to all double-cropped treatments. Ordination of overall nema-tode communities yields similar aggregations, probably due to the in-fluence of the plant-parasitic species. Community ordination tech-niques are not as effective for analysis of communities within a single field for a single cropping season as they are for analysis of com-munities in widely separated fields or vegetation types.