Plant species diversity enhances soil gross nitrogen transformations in a subtropical forest, southwest China

1. Plant species diversity (PSD) regulates ecosystem structure and functions, and is a key issue we need to consider when design vegetation restoration projects. Increasing PSD has been shown to promote or decrease soil nitrogen (N) availability, but the underlying mechanisms have not been well explored. 2. Here, 45 plots with the Shannon-Weiner indices ranging from 0.15 to 3.57 were selected in a subtropical forest to explore the effect of PSD on soil N transformations. 3. Higher PSD significantly enhanced the rates of gross N mineralization, gross nitrification, microbial N immobilization, net N mineralization, net nitrification and the contents of soil total N and inorganic N. Structural equation modelling showed that PSD indirectly impacted gross N transformations via its roles in regulating soil organic matter, mineral and microbial traits. Higher PSD stimulated gross N mineralization and nitrification mainly via its positive effects on microbial biomass content and gene abundances of chiA, archaeal or bacterial amoA, while increased microbial N immobilization mainly due to its stimulation of soil organic matter. 4. Synthesis and applications. Our findings highlight the crucial role of PSD in stimulating soil N availability and provide a mechanistic understanding which can be integrated into Earth system models to better predict soil N availability and C sequestration in response to PSD.