Objective Under the background of increasing atmospheric nitrogen deposition, this study aimed to determine whether ectomycorrhizal fungi (EMF) regulate leaf nitrogen allocation and thereby influence photosynthetic capacity and nitrogen use efficiency in Pinus massoniana seedlings.

Method In this study, we investigated the effects of EMF inoculation on leaf nitrogen allocation in P. massoniana seedlings under varying nitrogen deposition levels. Seedlings were inoculated with two EMF species, Pisolithus tinctorius (Pt) and Suillus grevillei (Sg), and exposed to four nitrogen addition treatments: 0 kg·N ha⁻¹a⁻¹ (N0), 30 kg·N ha⁻¹a⁻¹ (N30), 60 kg·N ha⁻¹a⁻¹ (N60), and 90 kg·N ha⁻¹a⁻¹ (N90).

Result 1) Nitrogen addition significantly increased mycorrhizal colonization rate, seedling height, stem diameter, and dry biomass of both above and below ground parts, with the highest values observed under moderate N deposition (N60). Leaf nitrogen concentration per unit mass (N_mass), per unit area (N_area), photosynthetic nitrogen use efficiency (PNUE), and maximum photosynthetic rate (P_max) exhibited a unimodal response initially increasing and then decreasing. Across nitrogen treatments, all these parameters were significantly higher in EMF inoculated plants than in non-inoculated controls. 2) EMF inoculation, nitrogen addition, and their interaction significantly increased nitrogen allocation to water-soluble and membrane proteins in leaves, while reducing the proportion allocated to cell wall proteins, indicating a significant upregulation of nitrogen metabolism in seedlings. 3) Moderate nitrogen addition (N60) also promoted nitrogen allocation to photosynthetic apparatus, enhanced Rubisco activity, and improved carboxylation efficiency and electron transport rate, thereby markedly boosting photosynthetic carbon assimilation.

Conclusion EMF inoculation modulated leaf nitrogen allocation strategies in response to varying nitrogen availability, highlighting their role in enhancing plant adaptability to nitrogen deposition. These findings provide theoretical support for predicting EMF mediated physiological adjustments of leaf traits under scenarios of increasing atmospheric nitrogen deposition.