Regulatory effects of ectomycorrhizal fungi on the morphological construction of Pinus sylvestris var. mongolica seedlings under exogenous nitrogen input

This study investigated how ectomycorrhizal (ECM) fungal inoculation affects the rhizosphere microenvironment and growth of Pinus sylvestris var . mongolica seedlings under different nitrogen (N) addition levels. Results showed microbial diversity and biomass exhibited threshold responses to N. Under ECMF inoculation, low N (LN) increased bacterial diversity (Chao1 index +13.40 %) and microbial biomass phosphorus (+139.20 %), whereas high N (HN) only marginally increased diversity and decreased microbial biomass carbon (-41.4 %). LN inoculation also increased total soil C and P, alleviated microbial C limitation, and promoted seedling morphology (e.g., specific root length +58.97 %, seedling height +58.58 %). In contrast, HN inhibited root development and exacerbated the effects of N limitation. Without inoculation, HN resulted in higher bacterial diversity and soil nutrient content than did LN. Path analysis identified N addition and inoculation as key positive drivers of aboveground growth, with root morphology showing negative feedback. In conclusion, ECMF inoculation benefits seedling growth by enhancing microbial diversity and P availability under low N conditions, but high N causes C N imbalance and suppresses mycorrhizal benefits, providing theoretical support for mycorrhizal application in semi-arid areas under N deposition. Consequently, based on an identified threshold of ≈6 g·m⁻²·a⁻¹ for N addition, we recommend ECMF inoculation only under low-N conditions to ensure seedling morphological formation. This practice should be avoided under high-N conditions to prevent the loss of symbiotic benefits and a decline in ecological adaptability.