Effects of nitrogen addition on ecosystem-level carbon fluxes and water use efficiency are more dependent on early growing season precipitation than on annual precipitation in a semi-arid meadow steppe

Precipitation and anthropogenic nitrogen (N) may have profound impacts on ecosystem carbon (C) fluxes and the plant water use efficiency (WUE), particularly in semi-arid salinized and alkalized grasslands that were limited by both N and water availability. To better understand the mechanism of N and precipitation effects on C fluxes and plant WUE, in 2015, we established a randomized block-design N addition experiment, including five N levels (0, 5, 10, 20, 40 g N m−2 yr−1) in a meadow steppe (dominated by Leymus chinensis (Trin.) Tzvel.) of northeast China. Between 2016–2019, we investigated how N addition, combined with different amounts of annual and early growing season precipitation availability, modulated ecosystem C fluxes (NEE, net ecosystem CO2 exchange; ER, ecosystem respiration; GEP, gross ecosystem production), plant WUE, including leaf-scale long-term intrinsic WUE (iWUE) inferred from 13C composition and ecosystem-scale WUE (WG, WG = GEP/ET), as well as related biotic and edaphic variables. We found that: (i) N supply decreased NEE and increased ER, GEP, and WG in a nonlinear pattern; (ii) N addition increased iWUE and thus WG; and (iii) higher early growing season precipitation, in contrast to annual precipitation, magnified the effect of N on NEE, ER, GEP, and WG, suggesting that N effects on NEE, ER, GEP, and WG depend more on early growing season precipitation than annual precipitation. The present study improves our understanding of the coupled mechanisms of future global C, N, and water dynamics shaped by global climate change.