Continuous Cropping Alters Soil Microbial Community Assembly and Co-Occurrence Network Complexity in Arid Cotton Fields

This study examines the impact of continuous cropping (short-term: 1–8 years; medium-term: 9–15 years; long-term: 16–30 years) on soil microbial community diversity, co-occurrence networks, and assembly processes in Xinjiang’s cotton region, a globally recognized arid zone. The results are as follows. Soil physicochemical analyses showed that as continuous cropping duration increased, soil organic matter and total nitrogen significantly decreased, whereas available phosphorus and potassium increased, and the soil’s aggregate structure degraded. Microbial community analysis indicated that long-term continuous cropping notably increased the richness of bacterial species (Chao1 index) and altered fungal communities’ diversity and composition, especially increasing the relative abundance of Cladosporium and Alternaria in the long term (GY30). Co-occurrence network analysis revealed higher complexity in bacterial and fungal networks in the short term. As cropping duration increased, bacterial network complexity significantly decreased, while fungal networks partially recovered in the long term, indicating greater fungal adaptability to environmental changes. Assembly process analysis revealed that the assembly of bacterial and fungal communities was jointly regulated by stochastic and deterministic processes, but with increasing cropping duration, deterministic processes weakened while stochastic processes intensified. Soil available phosphorus, potassium, and pH were identified as key factors influencing microbial community succession and assembly. This study highlights the significance of co-occurrence networks and assembly processes for understanding the dynamics of continuous cropping’s impact on soil microbial communities, offering a theoretical foundation for improving agricultural management.