Introducing Legumes into Wheat–Maize Rotation Complicates Soil Microbial Co-Occurrence Network and Reduces Soil Allelochemicals in Succeeding Wheat Season

Increasing species richness through rotation is considered a promising measure to enhance agroecosystem functions and services. However, the legacy effects of introducing legumes into a wheat–maize rotation in the North China Plain on soil microecology, especially the soil metabolome, in the succeeding wheat season have not been elucidated. This study established three cropping systems: (1) a continuous winter wheat–summer maize rotation (M), (2) a winter wheat–summer peanut (summer maize) rotation (PM), and (3) a winter wheat–summer soybean (summer maize) rotation (SM). The soil physicochemical properties, microbial communities, and metabolomes were analyzed at the stage of the succeeding wheat crop. Introducing peanuts or soybeans into a wheat–maize rotation significantly reduced the soil bacterial abundance and increased the soil fungal Shannon index. This rotation adjustment had a substantial impact on the structure and taxa composition of the soil microbial community. Crop diversification increased the number of total edges, the average degree, and the average number of neighbors in the soil microbial co-occurrence network. Different crop rotations significantly affected the soil metabolic profiles in the positive and negative ion modes. Crop diversification significantly reduced the abundance of coumarin and coumaric acid in the soils. In conclusion, introducing peanuts or soybeans into a wheat–maize rotation could increase the soil fungal community diversity, change the soil microbial community structure and taxa composition, increase the complexity of the soil microbial ecological network, and reduce the abundance of soil allelochemicals. Our study demonstrated the continuity of the impact of crop rotation on soil ecology, and revealed the ecological advantages of crop diversification from the perspective of soil microbiology and metabolomics.