Abstract: Nutrients supply and carbon input affect the stability of soil organic carbon in forests. In this study, we examined the effects of nitrogen deposition, coupling exogenous nutrients and carbon sources on soil organic carbon mineralization and mineral nitrogen release in a Chinese fir (Cunninghamia lanceolata) plantation in hilly mountainous area at the later stage of the treatments. Soils were treated with following conditions: no addition (CK); N addition alone \[N\]; N and carbon additions \[N+C\]; N and non-N nutrient additions \[N+nutrient\]; N, non-N nutrients and carbon additions \[N+both\]. The results showed that the total mineralization of soil organic carbon under different treatments increased with time extension during 720 days incubation, while the mineralization rate gradually decreased. After 360 days incubation, the contents of soil NH₄⁺-N, NO₃^--N, and mineral nitrogen under all the treatments were higher than those under CK. After 720 days incubation, the contents of NO₃^--N and mineral nitrogen were the highest under the \[N\] treatment, followed by the treatments \[N+nutrient\], \[N+both\], and \[N+C\]. The activity of β-glucosidase was the highest under the \[N+both\] treatment, followed by \[N+C\], CK, and \[N\]. The activity of β-N-acetyl-glucosaminidase was the highest under the \[N+both\] treatment. The activity of acid phosphatase was higher under the treatment of \[N+both\] and \[N+C\] than that under the \[N\] treatment and CK. Under the background of nitrogen deposition, long-term input of non-nitrogen nutrient and carbon into soils will promote soil organic carbon decomposition and thus weaken soil carbon sequestration in forests.

Key words: Cunninghamia lanceolata, nitrogen deposition, organic carbon stability, enzyme activity.