Responses of soil carbon and nitrogen mineralization to nitrogen addition in a semiarid grassland: The role of season

• N addition enhanced soil N immobilization over all seasons. • N addition impacts on soil C sequestration depended upon seasons. • Factors regulating the soil C min and N min rates in response to N addition were seasonally variable. • Soil C min and N min rates became decoupled with N addition for most seasons. Elevated atmospheric N deposition can profoundly alter soil carbon (C) mineralization (C min ) and nitrogen (N) mineralization (N min ), which could severely impact long-term productivity of grassland ecosystem. However, little is known about how N addition, season, and their interaction affect soil C min and N min rates and their relationships by regulating soil abiotic and biotic factors. Here we investigated the seasonal variations in soil C min and N min rates and their relationship in response to multi-level N additions in a semiarid grassland in 2014–2015, and further identified direct and indirect pathways by which soil abiotic and biotic factors regulated these variations using structural equation modeling. We documented the statistically significant impacts of N addition and its interaction with season on soil C min rates. In contrast, only a significant seasonal effect on the soil N min rate was observed. Random forest analysis revealed that across all seasons, dissolved organic carbon (DOC), soil water content (SWC), catalase, urease, sucrase, microbial biomass carbon (MBC), soil organic carbon (SOC) to total nitrogen (TN) ratio, and TN were the most pivotal predictors of the soil C min rate. Comparatively, catalase, MBC, DOC, NO 3 – -N, urease, TN, NH 4 + -N, SWC, and the MBC to microbial biomass nitrogen (MBN) ratio were the most dominant drivers of the soil N min rate. SEM results indicated that the identified potential drivers that regulated the soil C min and N min rates in response to N addition varied seasonally. Additionally, N addition decoupled the soil C min and N min rates, which was a consistent relationship among most seasons. In summary, our results show that, in this semiarid grassland, current N additions can enhance soil N immobilization across all seasons; however, its impacts on soil C sequestration were seasonally variable. These findings provide evidence that season and its interactions with elevated atmospheric N deposition have important implications for the grassland biogeochemical cycling.