Effects of Organic–Inorganic Nitrogen Deposition on Element Content and Stoichiometric Ratios in the Rhizospheric Soil of Torreya grandis

The deposition of atmospheric nitrogen (N) includes organic N (ON) and inorganic N (IN). The effect of varying ON:IN ratios on the elemental composition in the root zone of T. grandis remains poorly understood. In this study, N deposition with different ON:IN ratios was simulated to analyze its effects on soil organic carbon (C), total N (TN), total and available phosphorus (P), available potassium (K), exchangeable calcium, exchangeable magnesium, and C:N, C:P, and N:P ratios. The results showed that N deposition increased soil organic C and total N content when compared with the control group. Different N sources had varied effects on soil P, with high inorganic N inputs potentially exacerbating P depletion. Nitrogen addition significantly reduced available K and exchangeable calcium and magnesium. Nitrogen addition treatment at an ON:IN ratio of 9:1 resulted in elevated C:N and C:P ratios, while nitrogen addition at an ON:IN ratio of 7:3 resulted in a notably increased N:P ratio. In conclusion, N deposition in different forms significantly affects soil elemental contents and ecological stoichiometry in T. grandis by altering C-N accumulation and soil acidification. Monitoring the loss of calcium, magnesium, and K is essential, and soil nutrient management should be enhanced to sustain forest health amid increasing N deposition.