Seasonal precipitation regulates magnitude and direction of the effect of nitrogen addition on net ecosystem CO2 exchange in saline-alkaline grassland of northern China

Increased nitrogen (N) deposition and altered precipitation regimes have profound effects on carbon (C) flux in semi-arid grasslands. However, the interactive effects between N enrichment and precipitation alterations (both increasing and decreasing) on ecosystem CO2 fluxes and ecosystem resource use efficiency (water use efficiency (WUE) and carbon use efficiency (CUE)) remain unclear, particularly in saline-alkaline grasslands. A four-year (2018–2021) field manipulation experiment was conducted to investigate N enrichment and precipitation alterations (decreased and increased by 50 % of ambient precipitation) and their interactions on ecosystem CO2 fluxes (gross- ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem CO2 exchange (NEE)), as well as their underlying regulatory mechanisms under severe salinity stress in northern China. Our results showed that N addition and precipitation alteration alone did not significantly affect the GEP, ER and NEE. While the interaction of N addition and increased precipitation over the four years significantly improved the mean GEP and NEE by 24.9 % and 15.9 %, respectively. The interactive effects of N addition and increased precipitation treatment significantly stimulated the mean value of WUE by 39.1 % compared with control, but had no significant effects on CUE over the four years. Based on the four-year experiment, the magnitude and direction of the effects of N addition on the NEE were related to seasonal precipitation. Nitrogen addition increased the NEE under increased precipitation and decreased it during extreme drought. Soil salinization (pH and base cations) could directly or indirectly affect GEP and NEE via plants productivity, plant communities, as well as ecosystem resource use efficiency (WUE and CUE) based on structural equation model. Our results address lacking investigations of ecosystem C flux in saline-alkaline grasslands, and highlight that precipitation regulates the magnitude and direction of N addition on NEE in saline-alkaline grasslands.