Precipitation increase enhanced the positive effect of nitrogen addition on soil N2O emissions by promoting soil nitrogen transformation and plant productivity in saline-alkaline grassland of Northern China

Individual effects of continued increases in nitrogen (N) deposition and changes in precipitation have been reported to have profound effects on N cycling in grassland ecosystems. However, the response and regulatory mechanisms of soil nitrous oxide (N₂O) flux to N deposition under conditions of changing precipitation are still unclear, especially in saline-alkaline grasslands. A 3-yr (2021–2023) manipulative field experiment was carried out to investigate the effects of N addition and precipitation changes ( ± 50 % in ambient precipitation) in a saline-alkaline grassland of the agro-pastoral ecotone in Northern China. The results indicate that: (1) compared with the results of the control plots, N addition alone significantly increased soil N₂O flux by 107.0 %; changes in precipitation alone showed no significant effect on soil N₂O flux; and N addition combined with increased precipitation yielded a significant increase of 180.1 % in soil N₂O flux across the three years. (2) The positive effect of N addition on soil N₂O flux was increase exponentially with increasing precipitation. (3) N addition, rather than a change in precipitation, significantly increased the seasonal mean net nitrification rate (R_nit) by 191.8 %, the net N mineralization rates (R_min) by 181.7 %, and the gene abundance of ammonia-oxidising archaea (AOA) by 3 %. (4) Soil N₂O flux was significantly and positively correlated with above-ground primary productivity (ANPP), R_nit, and R_min; and the relative changes in soil N₂O flux and plants productivity induced by N addition were all increased with precipitation. (5) Plants characteristics and soil microbial N mineralization indirectly regulated the effects of N addition and precipitation alteration on soil N₂O flux. These observations highlight that increased precipitation can enhance the soil N₂O emissions under concurrent N deposition scenarios, and thus exacerbate global warming. This study provided a theoretical basis for the restoration and utilization of degraded grasslands in agro-pastoral ecotone grassland in Northern China.