Abstract: Aims The response of forest soil organic carbon (SOC) to global change has remained a prominent frontier issue in ecological research. Although numerous studies have demonstrated that nitrogen deposition can promote the accumulation and of forest SOC and enhance its stability, the impact of nitrogen deposition on the accumulation process of specific plant-derived components in SOC, such as lignin phenols, remains unclear. This knowledge gap limits our in-depth understanding of forest carbon cycling processes and their pivotal role in maintaining global carbon balance. Methods Based on the long-term (more than 20 years) nitrogen deposition experimental platform in Dinghu Mountain（112° 10′ E, 23° 10′ N）, this study systematically investigated the effects of long term nitrogen addition on the accumulation patterns of lignin phenols across different soil layers and their underlying mechanisms. Important findings The results indicated that: (1) Lignin phenol content significantly decreases with soil depth; Nitrogen addition increased lignin phenol content by 23.7% in the 0-10 cm soil layer but had no significant effect on the 10-30 cm layer. (2) Under the control treatment, the activity of C-degrading enzymes is a key factor influencing lignin phenol accumulation; Nitrogen addition inhibited C-degrading enzyme activityand weakened their regulatory effect on the accumulation of lignin phenols. However, lignin phenols were more effectively protected through physical and chemical binding with soil minerals, thereby significantly reducing their bioavailability. In summary, the impact of long-term nitrogen deposition on plant-derived carbon in tropical forests exhibits significant soil layer dependency, with its effects limited to promoting lignin phenol accumulation in the surface soil layer. long term high nitrogen input has altered the regulatory pathways of plant-derived carbon accumulation in tropical evergreen broadleaf forest soils, shifting from microbial degradation processes to physicochemical protection as the dominant mechanism. This study unveils a novel mechanism for maintaining the stability of tropical forest soil carbon pools under nitrogen deposition, providing crucial theoretical insights for predicting forest soil carbon dynamics.

Key words: lignin phenol accumulation, mineral physical and chemical protectio, soil organic carbon sequestration, nitrogen deposition, tropical forest