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)₂SO₄ 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)₂SO₄ 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.