【Objective】Anthropogenic nitrogen (N) deposition is a key issue in global-change research. As the two most critical nutrients for plant growth, nitrogen and phosphorus (P) regulate ecosystem functioning; however, elevated N deposition intensifies N-P imbalance in P-limited subtropical forest soils, strongly altering soil N cycling.【Method】A three-year experiment was conducted in a subtropical moso bamboo (Phyllostachys edulis) plantation to evaluate the effects of control (CK, 0 kg·hm-2· a-1), nitrogen addition (N, 120 kg·hm-2·a-1 (in terms of N, the same below) ), phosphorus addition (P, 120 kg·hm-2·a-1(in terms of P, the same below)), and combined N and P addition (NP, 120 kg·hm-2·a-1 + 120 kg·hm-2·a-1) on soil organic N mineralization across 0-20, 20-40 and 40-60 cm soil depths.【Result】The results showed that: (1) The activities of soil enzymes responsible for C and N transformations were found to be generally reduced under N and NP treatments but increased under P treatment. A significant effect of soil depth was identified, revealing a pronounced decreasing trend in enzymatic activities with greater depth; (2) Overall, N treatment and NP treatment significantly reduced microbial biomass C and N (MBC, MBN) and dissolved organic C and N (DOC, DON) in the 20-40 cm and 40-60 cm soil layers, except for the P treatment reducing DOC and MBC in the 20-40 cm soil layer. Also, P treatment had a trend of increasing MBC, MBN, DOC, and DON in all soil layers. Soil depth significantly affected the content of soil active C and N, with both gradually decreasing as soil depth increased. (3) N and NP additions exerted an inhibitory effect on organic N mineralization, whereas P addition showed a stimulatory trend. However, CK, N, P, and NP treatment differences were not statistically significant. Soil depth strongly affected mineralization rates: net N mineralization in the 0-20 cm layer was significantly higher than in the 20-40 cm and 40-60 cm layers. No significant difference was detected between the 20-40 and 40-60 cm layers. (4) Structural equation modeling (SEM) revealed that nitrogen addition and soil layer effects indirectly inhibit organic N mineralization by reducing β-glucosidase (BG) and DON; P addition indirectly promotes organic N mineralization by increasing DON.【Conclusion】In summary, in the N-rich and P-limited soil of a subtropical Phyllostachys edulis (moso bamboo) forest, short-term (three-year) N addition alone (N treatment) and combined N and P addition (NP treatment) exhibit an inhibitory trend on soil organic N mineralization, whereas P addition alone (P treatment) shows a promoting trend. Considering the vertical mobility of inorganic N in subtropical forest soils, further attention should be paid to the long-term effects of N and P additions on soil N mineralization across different soil layers. By elucidating the characteristics and underlying mechanisms of soil organic N mineralization in response to N and P additions and soil depth in a moso bamboo forest, this study provides data support for understanding soil N transformation under N deposition.