<p>Plantation forests can enhance terrestrial carbon (C) sequestration by increasing both plant biomass and soil organic C (SOC) stocks. Elevated atmospheric nitrogen (N) deposition has profoundly affected forest SOC budget by altering plant C inputs and microbial C utilization. Most studies have simulated N deposition through understory N addition (UN), concluding that increased N inputs promote SOC sequestration primarily through greater accumulation of particulate organic C (POC). However, the UN approach is criticized for neglecting the forest canopy-mediated processes. Here, we conducted a 3-year gradient N-manipulation experiment (0, 25, and 50&#xa0;kg N ha^–1 yr^–1) including both canopy N addition (CN) and UN treatments in a subtropical plantation forest, to assess how SOC dynamics respond to different N-addition approaches. The results revealed that the UN treatments led to greater SOC accumulation than CN treatments. Specifically, both UN rates significantly increased POC and total SOC concentrations, with only minor effects on mineral-associated organic C (MAOC). In contrast, the high‐rate CN treatment substantially increased MAOC without affecting POC or SOC concentrations. Compared to UN treatments, CN treatments significantly decreased litter C: N ratio and mitigated N‐induced soil acidification, thereby accelerating the microbial transformation of labile POC into more stable MAOC. These findings indicate that conventional fertilization experiments may overestimate the benefits of N deposition for SOC accumulation in forest ecosystems. Projections of future plantation forest soil C sequestration under elevated atmospheric N deposition should explicitly incorporate the buffering effects of the forest canopy.</p>