Nonlinear responses of ecosystem carbon fluxes and water-use efficiency to nitrogen addition in Inner Mongolia grassland

1. Nitrogen (N) deposition is a continuous process and likely to affect ecosystem carbon (C) and water fluxes in a nonlinear way. However, experimental evidence is still lacking because most previous studies on these impacts usually used two discrete levels of N treatment. 2. By a 12-year, 6-level N addition experiment in Inner Mongolia grassland, we found that the responses of C fluxes, including net ecosystem carbon exchange (NEE), gross ecosystem productivity (GEP), ecosystem respiration (ER), all exhibited nonlinear patterns with increasing N addition rate while that of evapotranspiration did not significantly change. As a result, the response of ecosystem water-use efficiency (EWUE) followed a similar pattern with NEE. 3. These N-induced changes in C fluxes were greatly affected by the distribution of precipitation among different stages of growing seasons and mainly driven by the alterations in biomass production rather than soil temperature and soil moisture. 4. This study highlights the importance of the nonlinearity of N addition impacts on ecosystem C fluxes, which should be incorporated into the global-C-cycling models for better predicting future C balance. Our results also have implications for the use of fertilization in restoring the degraded grasslands given that N addition can promote biomass production and ecosystem C uptake without additional water evapotranspiration but also change ecosystem composition.