Abstract: Soil labile carbon and nitrogen (C, N), the most important components in the nutrient cycle of farmland ecosystem, is closely related to C and N turnover rate and nutrient availability, while its content is significantly influenced by straw returning, and is regulated by micro-organism. However, the changes of soil labile C and N under the combined effects of straw returning and micro-organism need to be further studied. In this study, a microcosm incubation experiment was set up in which the whole nematode communities were extracted from fresh soil and reinoculated into the soil defaunated by 5 kGy dose gamma irradiation that left the microbial community largely intact. The soil was cultured for 154 days, and the content of labile C and N was determined at different stages within 154 days of culture period. Four treatments were included: straw mulching without nematodes, straw mulching with nematodes, straw incorporation without nematodes and straw incorporation with nematodes. The results showed that the inoculation rate of soil nematode communities could reach more than 70% in a relatively short period of time after inoculation, and all trophic groups were detected. Soil labile C and N contents increased or decreased stepwise with the increase of incubation time. The soil CO₂ emission rate, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) contents of the treatment with nematodes were greater than those of the treatment without nematodes, and soil dissolved organic carbon (DOC), dissolved organic nitrogen (DON), mineral nitrogen (MN) and ammonium nitrogen (NH₄⁺) contents were lower than those of the treatments without nematodes. Nitrate nitrogen (NO₃⁻) content of the treatment with nematodes was lower than that of the treatment without nematodes during 7 − 54 days, but was greater than that of the treatment without nematodes during 91 − 154 days. Throughout the incubation period, the magnitude of the effect of nematodes incubated under straw mulching on MBC was greater than that under straw incorporation, while the magnitude of the effect on NO₃⁻ was less than that under straw incorporation. These results help to elucidate the role of soil nematodes in regulating soil labile C and N turnover under straw returning conditions, and provide a scientific basis for C and N cycling in soil agroecosystems.