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

Because soil nematode populations play a key role in regulating the turnove r of microbial communities and respond to changes in environmental conditio ns, shifts in their abundance and composition can be useful indicators of s oil conditions. In this study nematode communities and other ecosystem vari ables were investigated over 7 yr under an annual (Zen mays) and a perennia l (Asparagus officinale) crop using three weed management practices (cultiv ation, herbicide application, mulching) which can be related to agricultura l intensification. Crop productivity and soil conditions did not change sig nificantly during the trial. All management practices influenced the nemato de 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-fe eding nematodes but both subsequently declined, which was coincident with e nhanced populations of top predatory nematodes. The apparent negative inter action between bacterial-feeding and predatory nematodes was also demonstra ted through the former being significantly (P < 0.001) negatively correlate d with soil carbon, bacterial mass and weed biomass and the latter being po sitively correlated with the same variables. Herbicide use did not exert an y consistent detrimental effects on nematode communities and the nematode f auna in the herbicide treated plots tended to have greater diversity (as in dicated by the Shannon-Weiner index) than that in many of the other plots. The effects of cultivation varied, but under the perennial crop the greates t number of total and bacterial-feeding nematodes were commonly at 5-10 cm depth in cultivated plots. While most treatments had relatively little gene ral 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 h as achieved some degree of equilibrium rather than monitoring only initial cropping cycles. (C) 1999 Elsevier Science Ltd. All rights reserved.