Abstract: The community characteristics of soil nematodes are an important basis for evaluating and indicating soil health. In this study, cotton fields with different continuous monocropping years (5, 10, 15, 20, and 25 years) were selected. High-throughput sequencing technology was used to explore the responses of soil properties and nematode communities to long-term continuous monocropping in cotton fields. The results showed that after 10-15 years, soil pH and soil conductivity increased significantly, and the contents of soil organic carbon, total nitrogen, available phosphorus (AP), available potassium, nitrate and soil microbial biomass carbon (MBC) decreased significantly. A total of 25 genera, 18 families, 7 orders and 3 classes of soil nematodes were identified. Among them, Helicotylenchus was the dominant genus. Parasitic nematodes were the dominant trophic groups, showing a trend of first decreasing and then increasing. Compared with other continuous monocropping years, plant parasitic nematodes increased by 9.1%-208.6% and Helicotylenchus increased by 392.0% under continuous monocropping for 25 years. With the increases of continuous monocropping duration, plant parasitic nematodes such as Tylenchorhynchus, Ditylenchus, Discopersicus, Mesocriconema, and Criconemoides were detected. After continuous monocropping for 15 years, soil nematode richness index and maturity index of free-living nematodes (MI) decreased significantly, PPI/MI increased significantly, and diversity index (Shannon) and Wasilewska index were the lowest. Soil AP and MBC were the main environmental factors affecting the changes of soil nematode communities. Our results suggested that, after 10-15 years continuous monocropping, soil nutrients were unbalanced, the diversity of soil nematodes was reduced, the stability of soil food network was deteriorated, and the parasitic nematodes were increased, which resulted in continuous monocropping obstacles in cotton.

Key words: cotton, continuous monocropping, soil nematode, trophic group