Understory vegetation restoration improves soil physicochemical properties, enzymatic activity, and changes diazotrophic communities in Cunninghamia lanceolata plantations but depends on site history

Aims Elucidating the mechanisms of understory vegetation response to forest management practices is critical to the impact of soil nutrient cycling. However, as common management practices, how thinning leading to the restoration of understory vegetation affects soil biochemical properties and microbial-mediated belowground processes and their main drivers remains unclear. Methods We investigated the differences of 16 S rRNA, nifH gene abundance, diversity and composition of vegetation restoration using a 5-year in situ experiment including Pinus massoniana- dominated forests (CK), virgin Cunninghamia lanceolata plantations (CF) and C. lanceolata plantations that have undergone a cycle of cutting and replanting (CS) in southern China. Results We found that understory vegetation restoration improved some soil physicochemical properties (e.g., AP) and enzyme activity. The abundance of nifH were higher in CF than in CK or CS. As a result of vegetation restoration, α-diversity of soil diazotrophic communities significantly increased in CF. These communities were clustered into different groups based on the forest type. The number of potential keystone species of diazotrophs was highest in CF, including several beneficial taxa (e.g., Paraburkholderia and Pelomonas ). Most of the bacterial functional groups related to soil nitrogen cycling were also significantly elevated in CF, such as nitrogen fixers. Partial least squares path modelling (PLS-PM) further showed that soil diazotrophic communities were clearly shaped by soil nutrient content. Conclusions Overall, these results highlight the effects of vegetation restoration on soil diazotrophic communities and clarify the contributions of potential for improving edaphic conditions in areas of southern China with acidic red soils.