Soil Chemistry and Stoichiometric Responses of Male and Female Torreya grandis to Nitrogen Deposition Under Salt Stress

Increased atmospheric nitrogen (N) deposition and soil salinization commonly co-occur in subtropical economic forests, and responses to these stressors differ between sexes in dioecious plants. In this study, we explored soil chemical and stoichiometric responses of male and female Torreya grandis to N deposition under salt stress by adopting a two-factor completely randomized design. The two factors were (1) plant sex (2-year-old grafted male and female seedlings of T. grandis) and (2) environmental treatment (four nitrogen deposition levels: low, moderate, and high N combined with salt stress, as well as a control without salt addition). We then determined the rhizosphere C, N, P, Ca, K, and Mg concentrations and their stoichiometric ratios. The results showed that all indicators were significantly affected by sex, nitrogen treatment and their interaction (p < 0.0001). Males maintained significantly higher soil C and N levels than females across all treatments, with female soil N and C contents being 5.74–25.72% and 10.78–23.64% lower than those of males, respectively, and exhibiting far more stable stoichiometry. Moderate nitrogen deposition (SMN) increased male C:N, C:P and N:P ratios by 38.76%, 59.75% and 13.84%, distinctly lower than the 85.89%, 98.20% and 16.04% increments in females. In contrast, females had higher Mg content under all salt–nitrogen-combined treatments and greater stoichiometric plasticity, showing a 37.55% higher C:N ratio than males under low nitrogen addition (SLN). Moderate N relieved salt-induced nutrient limitation and alleviated salt-induced P immobilization, while excessive N (SHN) exacerbated stoichiometric imbalance: SHN elevated the N:P ratio by 109.73% in males and only 69.59% in females, narrowing the sexual difference in C:N ratio to 10.92% and triggering severe phosphorus limitation in male rhizosphere soil. Soil–leaf nutrient relationships and correlations differed greatly between sexes, indicating divergent nutrient adaptation strategies. Males adopted a Ca-dominated stress tolerance strategy, and females depended on Mg homeostasis for reproduction. This work provides a scientific basis for sex-specific nutrient regulation and sustainable cultivation of T. grandis under global change.