The Effects of Long-Term Land Use Changes on Bacterial Community Structure and Soil Physicochemical Properties in the Northeast Mollisol Region of China

Soil microorganisms are essential for maintaining the function and health of agricultural ecosystems. However, the responses of microbial communities to long-term changes in land use have been insufficiently explored. Hence, based on a 15 years of field experiments in the northeast Mollisol region of China, we applied the Illumina high-throughput sequencing technology to study the effects of different land use types, including conventional tillage (CT), bare land (BL), no tillage (NT), natural vegetation restoration (NVR), and afforestation (AF), on bacterial communities along the soil profile (0–5 cm, 5–10 cm, 10–20 cm, and 20–30 cm) and co-occurrence networks and identified their relationships with soil physicochemical properties. The findings indicated that the land use type as well as soil depth affected the diversity and structure of bacterial communities significantly. There was no marked difference in the diversity of bacterial communities between CT and NT at different soil depths, except for a depth of 20–30 cm. In NT, NVR, and AF, the relative abundance of Actinomycetota and Firmicutes was higher than that in CT. Conversely, CT showed a remarkably higher abundance of Proteobacteria and Acidobacteriota than BL, NT, NVR, and AF. Compared with CT and BL, increased stability and complexity of the community co-occurrence networks was identified for NT, NVR, and AF. Additionally, the diversity and composition of bacterial communities were correlated with the soil’s total nitrogen (TN), pH as well as total organic carbon (TOC). Our study revealed the potential mechanism by which long-term land use changes affected the distribution of soil bacterial communities, which was of high importance for sustainable development of agriculture and optimal management of land resources.