Responses of trophic groups of soil nematodes to residue application underconventional tillage and no-till regimes

A laboratory and a field study were conducted to monitor the increase in nu mbers and C-14 uptake of different trophic groups of soil nematodes in resp onse to residue addition and to examine the relative importance of bacteriv orous and fungivorous nematodes in conventional (CT) and no-till (NT) agroe cosystems. In general, soil nematode numbers increased more rapidly in resp onse to residue addition and became much more abundant (greater than five-f old) under laboratory conditions than in the field. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivo rous nematodes under both CT and NT regimes in the laboratory and field stu dies. A depth effect was observed in NT, but not in the CT treatment; this reflected the vertical residue distribution in both tillage regimes. Soil n ematodes were more abundant under NT than under CT in the field. The same p attern was observed at the beginning of the laboratory study but it reverse d later. The ratios of fungivorous-to-bacterivorous nematodes (FN-to-BN) we re not significantly different between CT and NT treatments at the beginnin g of the experiment. They were very low (less than 0.2) in both tillage reg imes, indicating that bacterivorous nematodes were relatively more importan t than fungivorous nematodes in both tillage agroecosystems. However, the F N-to-BN ratios increased with time after residue decomposition started, par ticularly in the CT treatment. This suggested that the relative importance of fungivorous nematodes increased with the progress of residue decompositi on. It was more pronounced in the CT treatment during the short period afte r residue application. In both the laboratory and field studies, the C-14 s pecific activity of soil nematodes and the ratio of C-14 bound in nematode biomass to total C-14 decayed in the experiment (reported elsewhere) were s ignificantly higher under CT than under NT, suggesting that soil nematodes use carbon more efficiently under CT than under NT. No significant differen ce of C-14 specific activity of soil nematodes was found between the two de pths under CT in both the studies; however, C-14 specific activity was sign ificantly higher in the 0-2.5 cm than in the 2.5-5.0 cm layer under NT in t he laboratory study. (C) 2000 Elsevier Science Ltd. All rights reserved.