【Objective】The atmospheric nitrogen deposition may significantly alter carbon cycling processes in terrestrial ecosystems. Soil particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) are the main components of SOC. The effect of nitrogen addition on their dynamic changes is critical for understanding and predicting the response and adaptation mechanisms of SOC in plantation forests under long-term nitrogen deposition.【Method】Taking the plantation of Populus deltoides cv. I-35 in the Dongtai Forest Farm, Jiangsu Province, experimental plots were established in May 2012 to simulate atmospheric nitrogen deposition artificially. Four nitrogen application levels were set, i.e. 0 g/(m²·a)(N₀), 5 g/(m²·a)(N₅),15 g/(m²·a)(N₁₅), and 30 g/(m²· a) (N₃₀). Each nitrogen treatment included four replicated plots, resulting in 16 plots. We collected soil, litter, and fine root samples from each plot at different nitrogen treatments in August 2023 (growing season) and January 2024 (non-growing season), respectively. The soil was analyzed for pH, ammonium nitrogen, nitrate nitrogen, total soil organic carbon, dissolved organic carbon, microbial biomass carbon, and other physicochemical properties, as well as for POC and MAOC fractions. Litter and fine roots were analyzed for total carbon, carbon-to-nitrogen ratio, and biomass. The changes in the concentrations of soil POC and MAOC under different nitrogen addition levels and their driving factors were investigated by two-way ANOVA, correlation and redundancy analysis (RDA).【Result】Nitrogen addition significantly reduced the content of MAOC overall but had no significant effect on the content of soil POC, except in the [0,10) cm (surface) soil during the non-growing season. Moreover, during the growing season, N₅, N₁₅, and N₃₀ treatments significantly reduced MAOC content in both surface and [20,40) cm (lower) soil layers. The effects on MAOC content in the middle soil layer were not significant. During the non-growing season, the N₁₅ treatment significantly reduced MAOC content in the lower soil layer, while the N₃₀ treatment significantly reduced MAOC content in the surface and [10,20) cm (middle) soil layers. All nitrogen treatments had no significant effect on POC during the growing season, while the N₃₀ treatment significantly increased the content of POC in the surface layer during the non-growing season. The contents of soil POC and MAOC generally decreased significantly with increasing soil depth. Regarding seasonal variation, the soil POC and MAOC were overall higher in the growing season than in the non-growing season, with significant changes observed in the middle and lower soil layers. Two-way ANOVA indicated that the interaction between nitrogen treatment and soil depth had a highly significant effect on MAOC but no significant effect on POC. Correlation analysis showed that MAOC was positively correlated with total carbon in leaf litter. Both POC and MAOC were significantly positively correlated with total soil organic carbon, dissolved organic carbon, microbial biomass carbon, ammonium nitrogen, soil moisture, and total nitrogen. In contrast, they were negatively correlated with pH, carbon-to-nitrogen ratio, and nitrate nitrogen. Redundancy analysis revealed that POC and MAOC were significantly positively correlated with microbial biomass carbon and total carbon, and significantly negatively correlated with nitrate nitrogen.【Conclusion】The findings of this study demonstrate that long-term nitrogen addition promotes the accumulation of particulate organic carbon (POC) while reducing the accumulation of mineral-associated organic carbon (MAOC). This suggests that prolonged global nitrogen deposition may negatively impact the accumulation and overall stability of soil organic carbon pools, which holds significant implications for the long-term storage of soil carbon in forest ecosystems.