Impacts of tillage, cover crop, and nitrogen on populations of earthworms, microarthropods, and soil fungi in a cultivated fragile soil

The impacts of tillage regime, cover crop, and nitrogen on various soil organisms inhabiting a fragile sandy soil (Brunosolic Gray Brown Luvisol) were determined. Soil samples were collected between 2000 and 2003 from a long-term tillage experiment, established in 1988 to determine the effect of tillage systems on yield of corn (Zea mays), soil quality, and weed populations. Populations of several of the soil organisms studied were significantly affected by one or more agronomic treatments. A single earthworm species, Aporrectodea turgida, was found in the experimental area. Worm populations were generally low and dominated by juveniles. Spring-sampled populations were significantly higher in no-till plots than in conventionally tilled plots. Fall-sampled populations were not affected as greatly by tillage, but were generally higher in no-till plots not receiving additional nitrogen or in plots overseeded with a rye (Secale cereale) cover crop. Soil microbial biomass, as represented by extractable soil DNA, was higher in the spring than in the fall. Populations of the soilborne stramenopile Pythium were generally higher in conventionally tilled plots, and were increased by a rye cover crop. Higher rates of nitrogen increased populations of total soil fungi but nitrogen had little effect on prostigmatid or cryptostigmatid mites; prostigmatid populations were generally higher in no-till plots. Spring populations of mesostigmatid mites were higher in plots with a rye cover crop than in plots without an overwintering plant cover. Conventional tillage stimulated populations of astigmatid mites during periods of high rainfall. Collembola populations were dominated by the families Onychiuridae and Isotomidae, but neither was greatly affected by any tillage treatment. Principal component analysis showed that populations of A. turgida and soil aggregation tended to be positively associated with one another, but that variations in populations of Onychiuridae springtails, prostigmatid mites, and Pythium tended not to be associated with changes in other variables. Overall, effects of tillage treatments on soil organisms were found to differ from previous reports in several respects, suggesting that soil type may impose conditions that over-ride the impacts of agronomic cultivation systems on populations of soil organisms.