Abstract: Longterm fertilization can change soil fertility and microbial characteristics, and indirectly alter soil nematode community composition. The aim of this study was to analyze the effects of optimized chemical fertilizer application combined with organic materials on soil nematode community structure and the correlation between soil nematode community and soil properties and extracellular enzyme activities, and to explore the responses of soil nematode community composition to soil fertility. There were five treatments, including CK (no fertilization), CF (chemical fertilizer), RF (reduced chemical fertilizer), RFS (reduced chemical fertilizer + straw returning) and RFMS (reduced chemical fertilizer + organic fertilizer + straw returning), in theexperiment. The results showed that a total of 33 nematode genera were identified. Bacterialfeeding nematodes and phytophagous nematodes were the dominant nutritional groups in all treatments. The abundance of soil nematodes in different treatments followed an order of RFMS >RFS >RF >CF > CK. The abundance of soil nematodes in RFMS and RFS treatments was significantly higher than that in other treatments (P<0.05). Compared with CK, the abundance of bacterial-feeding nematodes and omnivorous/predatory nematodes was increased and the reproduction of phytophagous nematodes was inhibited by single chemical fertilizer application and chemical fertilizer + organic materials application. The Wasilewska index (WI), maturity index (MI), structure index (SI), and enrichment index (EI) of RFMS treatment were 1.42, 1.35, 67.3 and 70.94, respectively, which were higher than those of the other treatments. Compared with CK treatment, RFS and RFMS treatments significantly reduced the ratio of plant parasite indexPPI) to MI. The abundance of soil nematodes and bacterial-feeding nematodes were positively correlated with soil organic matter content, total nitrogen content, and soil extracellular enzyme activities. Results of redundancy analysis showed that soil total phosphorus, organic matter, and total nitrogen had great impacts on soil nematode community composition. Therefore, reduced chemical fertilizer application combined with organic materials can help the reproduction of nematodes, enhance nematode species diversity, enrich the structure of soil nematode food web, and reduce the degree of disturbance by enhancing the stability of nematode food web, which would thus be an important option to maintain soil biological health.

Key words: reduced chemical fertilizer application, organic material,  soil nematode, soil extracellular enzyme activity.