Objective The process and mechanism of nitrogen deposition influencing wetland soil carbon mineralization have considerable uncertainty. This study aimed to clarify the pathways and mechanisms of nitrogen additions driving carbon mineralization in alpine meadow soils.

Method This study established four treatments such as control (0 g/(m²·year)), low (5 g/(m²·year)), medium (10 g/(m²·year)) and high (15 g/(m²·year)) nitrogen additions in the Poa attenuata communities in Napahai Alpine Meadow. We identified the direct and indirect effects of nitrogen-deposition-induced changes in soil microbial biomass carbon and nitrogen, enzyme activities and physicochemical properties on carbon mineralization.

Result (1) In contrast to control, low and medium level of nitrogen additions significantly increased the soil carbon mineralization rates by 22.84% and 53.15%, respectively. In contrast, high nitrogen addition had no significant effect on carbon mineralization rate. (2) The activities of soil urease, sucrase and catalase increased first and then decreased with increased nitrogen addition level; their maximum values were 34.78%, 22.46% and 70.47%, respectively in the medium nitrogen treatment. However, the nitrogen additions had no significant effects on the activity of acid phosphatase. The changes in soil urease, sucrase and catalase activities under nitrogen deposition significantly affected the carbon mineralization rates. The explanatory rates of urease, sucrase and catalase activities for carbon mineralization variations were 75.2%, 82.4%, and 82.8%, respectively. (3) The medium and high level of nitrogen additions significantly reduced soil pH, while nitrogen additions significantly increased the values of soil water, total organic carbon, microbial biomass carbon, total nitrogen, hydrolyzable nitrogen and ammonium nitrogen. The soil carbon mineralization rates under nitrogen depositions were significantly negatively correlated with pH, but significantly positively correlated with microbial biomass carbon, total nitrogen and hydrolyzable nitrogen. (4) The structural equation model indicated that the changes of soil enzyme activity under nitrogen additions have a significant direct effect on carbon mineralization. The changes of soil pH, microbial biomass carbon, and nitrogen components (total and hydrolyzable nitrogen) may indirectly regulate carbon mineralization by mediating soil enzyme activity.

Conclusion Nitrogen deposition enhances carbon mineralization in alpine meadow soils by increasing soil nitrogen nutrients and microbial biomass carbon, thereby enhancing enzyme activity. However, the soil acidification induced by nitrogen deposition inhibits the carbon mineralization rate.