Mycorrhizal tree composition regulates subtropical forest soil carbon and nitrogen pools through shifts in soil stoichiometry and inorganic nitrogen partitioning

Mycorrhizal tree composition is a key functional trait associated with forest soil biogeochemistry, yet how variation in arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree proportions relates to soil carbon (C) and nitrogen (N) cycling remains unclear. We investigated 12 subtropical forest stands in Huangshan, eastern China, spanning a natural mycorrhizal-composition gradient, and combined soil physicochemical measurements with metagenomic analyses of microbial communities and potential C and N cycling functions. Increasing AM tree proportion was associated with higher NO 3 − −N/NH 4 + − N ratio and lower soil C/N, indicating a more nitrate-rich and relatively N-enriched soil environment. Additionally, microbial community structure changed significantly across the gradient. Metagenomic analyses revealed selective functional reorganization, including lower lignin and chitin degradation, higher methanogenesis, and enhanced denitrification potential under greater AM proportion. Co-occurrence network analysis identified five microbial modules, which showed contrasting associations with soil organic carbon (SOC), total nitrogen (TN), inorganic N partitioning, and C and N cycling functional potential. Partial least squares path modeling indicated relatively small negative total associations of AM tree proportion with SOC (−0.24) and TN (−0.18), mainly through indirect pathways involving soil properties and microbial community characteristics. Overall, mycorrhizal tree composition was associated with soil C and N pools primarily through inorganic N partitioning, soil C:N stoichiometry, and microbial community structure rather than strong direct associations with SOC and TN. These findings identify inorganic N partitioning and microbial community organization as key links between AM–ECM tree composition and soil C and N pools.