Background and Aims <p>Nitrogen (N) deposition and precipitation change profoundly impact forest carbon (C) cycling processes. However, how soil organic C (SOC) storage responds to their combined effects remains unclear.</p> Methods <p>We investigated the integrated effects of N addition and precipitation reduction (Pr) on SOC storage fractions, including particulate organic C (POC) and mineral-associated organic C (MAOC), through a 6-year field experiment using a complete two-factor randomized block design. In the experiment, N addition was set at 0, 30 and 60&#xa0;kg N ha⁻¹&#xa0;yr⁻¹, while Pr was implemented as reductions of 0, 30% and 60% in a subtropical forest.</p> Results <p>Results showed that the dynamics and driving mechanisms of SOC storage fractions varied with soil depth. At 0–10&#xa0;cm depth, POC and MAOC storages were primarily enhanced by N addition alone, with POC benefiting from increased forest litter C and inhibited microbial decomposition, while MAOC was promoted through cation-bridging mechanisms (e.g., Ca²⁺ and Mg²⁺). At 10–30&#xa0;cm depth, MAOC storage increased under high-rate (60&#xa0;kg&#xa0;ha⁻¹&#xa0;year⁻¹) N addition combined with high-rate (60%) Pr, mainly due to reductions in fine root biomass and increases in gram-negative bacteria and arbuscular mycorrhizal fungi, whereas POC storage remained largely unchanged over the 6-year treatment period.</p> Conclusions <p>These findings highlight the critical roles of N deposition and its interactions with precipitation reduction in regulating SOC storage, providing new insights for accurately assessing soil C sequestration potential under N deposition and climate change in subtropical forests.</p>