Responses of soil nematode populations, community structure, diversity and temporal variability to agricultural intensification over a seven-year period

Because soil nematode populations play a key role in regulating the turnover of microbial communities and respond to changes in environmental conditions, shifts in their abundance and composition can be useful indicators of soil conditions. In this study nematode communities and other ecosystem variables were investigated over 7 yr under an annual (Zea mays) and a perennial (Asparagus officinale) crop using three weed management practices (cultivation, herbicide application, mulching) which can be related to agricultural intensification. Crop productivity and soil conditions did not change significantly during the trial. All management practices influenced the nematode fauna but the greatest long-term effects were from sawdust mulching. In the mulched plots there was an initial flush of both total and bacterial-feeding nematodes but both subsequently declined, which was coincident with enhanced populations of top predatory nematodes. The apparent negative interaction between bacterial-feeding and predatory nematodes was also demonstrated through the former being significantly (P<0.001) negatively correlated with soil carbon, bacterial mass and weed biomass and the latter being positively correlated with the same variables. Herbicide use did not exert any consistent detrimental effects on nematode communities and the nematode fauna in the herbicide treated plots tended to have greater diversity (as indicated by the Shannon–Weiner index) than that in many of the other plots. The effects of cultivation varied, but under the perennial crop the greatest number of total and bacterial-feeding nematodes were commonly at 5–10 cm depth in cultivated plots. While most treatments had relatively little general effect on the composition of the nematode fauna over the study period, several important specific effects were only apparent after at least 3 yr. Thus to effectively evaluate the relative effects of different agricultural practices in the long-term it is necessary to sample until the ecosystem has achieved some degree of equilibrium rather than monitoring only initial cropping cycles.