The response of soil respiration to different N compounds addition in a saline-alkaline grassland of northern China

The increase in atmospheric nitrogen (N) deposition has profound effects on soil respiration (SR). However, the responses of SR to the addition of different N compounds, particularly in saline-alkaline grasslands remain unclear. A 3-year controlled field experiment was conducted to investigate the responses of SR to different N compounds (NH4NO3, (NH4)(2)SO4 and NH4HCO3) during the growing seasons in a saline-alkaline grassland located in the agro-pastoral ecotone of northern China. Our results demonstrated that SR showed a bimodal pattern and a significant interannual difference that was regulated by air or soil temperature and precipitation. Nitrogen addition had a significant effect on SR, and the effect of N addition on SR varied yearly, which was related to seasonal precipitation. The mean SR across 3 years (2017-2019) was increased by 19.9%, 13.0% and 16.6% with the addition of NH4NO3, (NH4)(2)SO4 and NH4HCO3, respectively. The highest effect of NH4NO3 addition on SR across 3 years was ascribed to the highest aboveground net primary production, belowground net primary production (BNPP) and soil NO3- concentrations. SR (C loss) was significantly increased while plant productivity (C input) did not significantly change under NH4HCO3 addition, indicating a decrease in C sequestration. In addition, BNPP was the main direct factor influencing SR in this saline-alkaline grassland, and soil salinization (e.g. soil base cations and pH) indirectly affected SR through soil microorganisms. Notably, NH4NO3 addition overestimated the response of SR to N addition, and different N compounds should be considered, especially in saline-alkaline grassland.