A laboratory and a field study were conducted to monitor the increase in numbers and C-14 uptake of different trophic groups of soil nematodes in response to residue addition and to examine the relative importance of bacterivorous and fungivorous nematodes in conventional (CT) and no-till (NT) agroecosystems. In general, soil nematode numbers increased more rapidly in response to residue addition and became much more abundant (greater than five-fold) under laboratory conditions than in the field. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivorous nematodes under both CT and NT regimes in the laboratory and field studies. A depth effect was observed in NT, but not in the CT treatment; this reflected the vertical residue distribution in both tillage regimes. Soil nematodes were more abundant under NT than under CT in the field. The same pattern was observed at the beginning of the laboratory study but it reversed later. The ratios of fungivorous-to-bacterivorous nematodes (FN-to-BN) were not significantly different between CT and NT treatments at the beginning of the experiment. They were very low (less than 0.2) in both tillage regimes, indicating that bacterivorous nematodes were relatively more important than fungivorous nematodes in both tillage agroecosystems. However, the FN-to-BN ratios increased with time after residue decomposition started, particularly in the CT treatment. This suggested that the relative importance of fungivorous nematodes increased with the progress of residue decomposition. It was more pronounced in the CT treatment during the short period after residue application. In both the laboratory and field studies, the C-14 specific 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 significantly higher under CT than under NT, suggesting that soil nematodes use carbon more efficiently under CT than under NT. No significant difference of C-14 specific activity of soil nematodes was found between the two depths under CT in both the studies; however, C-14 specific activity was significantly higher in the 0-2.5 cm than in the 2.5-5.0 cm layer under NT in the laboratory study. (C) 2000 Elsevier Science Ltd. All rights reserved.