Nematode communities in organically and conventionally managed agricultural soils

Interpretation of nematode community indices requires a reference to a rela tively undisturbed community. Maturity and trophic diversity index values w ere compared for five pairs of certified organically and conventionally man aged soils in the Piedmont region of North Carolina. Available nitrogen (ni trate, ammonium) was estimated at various lag periods relative to times of sampling for nematode communities to determine the strength of correlative relationship between nematode communities and nitrogen availability. Soils were sampled six times yearly in 1993 and 1994 to determine the best time o f year to sample. Maturity values for plant parasites were greater in organ ically than conventionally managed soils, and differences between managemen t systems were greater in fall than spring months. However, other maturity and diversity indices did not differ between the two management practices. Differences in crop species grown in the two systems accounted for most dif ferences observed in the community of plant-parasitic nematodes. Indices of free-living nematodes were correlated negatively with concentrations of am monium, whereas indices of plant-parasitic nematodes were correlated positi vely with concentrations of nitrate. Due to the similarity of index values between the two systems, organically managed soils are not suitable referen ce sites for monitoring and assessing the biological aspects of soil qualit y for annually harvested crops.