Low Rather Than High Level Nitrogen Additions Accelerate Carbon Release Process and Inhibit Recalcitrant Carbon Allocation via Stirring Soil Enzymatic Activities in Plateau Meadows

It remains uncertain about the pathway by which atmospheric nitrogen (N) deposition mediates soil carbon (C) source-sink process. This study was initiated to identify the contribution of soil enzymatic activities and nutrient availabilities to C release process (respiration and mineralization), accumulation, and allocation under the N deposition.We established four treatments including control (0 added), low (20 kg N ha^–1 yr^–1), medium (40 kg N ha^–1 yr^–1), and high (60 kg N ha^–1 yr^–1) levels of N additions in meadow wetlands on the Qinghai-Tibet Plateau.We found that the size of soil respiration, C mineralization, and enzymatic activities increased through low-level N additions (17.1–62.5%) and decreased through high-level additions (21.2–52.9%) in contrast to the control. The concentration of recalcitrant organic C (ROC) declined in low-level additions (19.3%) but increased in medium (47.1%) and high (88.6%) additions. Microbial biomass C (MBC) level increased under low (88.1%) and medium (28.6%) additions, while it decreased under high additions (31%). Furthermore, the ratios of LOC/ROC (226.2%) and MBC/ROC (136.4%) increased in low level additions, whereas they decreased in high additions (37.2–63.6%). The contribution of enzymatic activities (59.7–71.2%) to C release and allocation was higher than that of C and N pools (4.7–16.6%).We can conclude that low rather than high level N additions accelerate C release process and reduce recalcitrant C allocation via stirring soil enzymatic activities. The findings are helpful to understand the mechanism of N deposition mediating C source-sink process in alpine meadows.