Differential Responses and Controls of Soil CO2 and N2O Fluxes to Experimental Warming and Nitrogen Fertilization in a Subalpine Coniferous Spruce (Picea asperata Mast.) Plantation Forest

Emissions of greenhouse gases (GHG) such as CO2 and N2O from soils are affected by many factors such as climate change, soil carbon content, and soil nutrient conditions. However, the response patterns and controls of soil CO2 and N2O fluxes to global warming and nitrogen (N) fertilization are still not clear in subalpine forests. To address this issue, we conducted an eight-year field experiment with warming and N fertilization treatments in a subalpine coniferous spruce (Picea asperata Mast.) plantation forest in China. Soil CO2 and N2O fluxes were measured using a static chamber method, and soils were sampled to analyze soil carbon and N contents, soil microbial substrate utilization (MSU) patterns, and microbial functional diversity. Results showed that the mean annual CO2 and N2O fluxes were 36.04 ± 3.77 mg C m−2 h−1 and 0.51 ± 0.11 µg N m−2 h−1, respectively. Soil CO2 flux was only affected by warming while soil N2O flux was significantly enhanced by N fertilization and its interaction with warming. Warming enhanced dissolve organic carbon (DOC) and MSU, reduced soil organic carbon (SOC) and microbial biomass carbon (MBC), and constrained the microbial metabolic activity and microbial functional diversity, resulting in a decrease in soil CO2 emission. The analysis of structural equation model indicated that MSU had dominant direct negative effect on soil CO2 flux but had direct positive effect on soil N2O flux. DOC and MBC had indirect positive effects on soil CO2 flux while soil NH4+-N had direct negative effect on soil CO2 and N2O fluxes. This study revealed different response patterns and controlling factors of soil CO2 and N2O fluxes in the subalpine plantation forest, and highlighted the importance of soil microbial contributions to GHG fluxes under climate warming and N deposition.