Abstract: Soil organic carbon (SOC) in forest ecosystems is a crucial component of the global carbon cycle. Under the persistent intensification of global nitrogen deposition, the effects of nitrogen addition on forest SOC and its components—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—have attracted widespread attention in ecology. However, the effects of nitrogen addition vary significantly under different soil pH conditions, and a systematic integrative framework explaining the underlying mechanisms remains lacking. This article systematically synthesizes the patterns of nitrogen addition effects on SOC, POC, and MAOC along a soil pH gradient (acidic: pH < 5.5; weakly acidic to neutral: pH 5.5–7.5; alkaline: pH > 7.5) and analyzes the differential mechanisms through three pathways: "carbon input-carbon output-stabilization processes." The study finds that in acidic forest soils, nitrogen addition primarily exhibits a positive effect by promoting net SOC accumulation—enhancing plant carbon inputs and suppressing soil carbon decomposition to reduce carbon output in the short term, while in the long term, inhibiting carbon output serves as the core pathway driving the accumulation of both SOC and POC. Concurrently, nitrogen addition can increase MAOC content by strengthening mineral–organic associations and slowing MAOC decomposition rates. In weakly acidic to neutral forest soils, the effects of nitrogen addition tend to balance out: short-term increases in carbon input and output may occur simultaneously, whereas in the long term, both may decline, leading to an overall weak response of SOC. Within this context, POC content often increases due to enhanced carbon inputs or suppressed decomposition, whereas MAOC may decrease due to base cation leaching and reduced microbial necromass input. In alkaline forest soils, existing evidence suggests that the overall response of SOC to nitrogen addition is mostly insignificant, likely attributable to the offsetting effects of increased carbon inputs and enhanced decomposition processes. However, clear research gaps remain regarding the response patterns and regulatory mechanisms of POC and MAOC in these systems. Future research should strengthen systematic exploration of soil carbon cycling processes in forest ecosystems under alkaline soil conditions, integrating multi-scale observations and mechanistic models to deepen the understanding of carbon-nitrogen coupling mechanisms mediated by plant-microbe-mineral interactions, thereby providing a scientific basis for accurately predicting forest carbon sink functions and optimizing nitrogen management strategies.

Key words: Soil organic carbon, Particulate organic carbon, Mineral-associated organic carbon, Forest ecosystem, Soil pH, Nitrogen addition