Asymmetric response of ecosystem carbon components and soil water consumption to nitrogen fertilization in farmland

Abstract Farmlands receive large amounts of nitrogen (N) through fertilization, which can affect ecosystem carbon (C) and water fluxes. However, the responses of ecosystem C components to N fertilization and their relationships with soil water consumption under N fertilization remain unclear. In the present study, we investigated the ecosystem C components and evapotranspiration under three N fertilizer rates application (0, 180 and 360 kg N ha−1 year−1, referred to as N0, N180 and N360, respectively) of two wheat varieties in three continuous growing seasons. We found that N fertilization increased all ecosystem C components as well as evapotranspiration. Excessive N application rates (N360) did not increase the net ecosystem C exchange (NEE) compared with that in the N180 treatment, which was mainly due to the larger increase in aboveground plant respiration (Rab) in the N360 treatment than the N180 treatment. The effects of N fertilization on NEE were larger than the effects on evapotranspiration, which led to an increase in water use efficiency (WUE). In addition, N fertilization increased soil respiration (Rs) by promoting both heterotrophic respiration (Rh) and root respiration (Ra), but a threshold effect of the N application rate on the Rs components was observed. Furthermore, the ecosystem C components showed different sensitivities to N fertilization, with Rab showing the highest sensitivity. Moreover, the changes in the C components were affected by the growing season and varieties, and mainly driven by biomass rather than by soil temperature or evapotranspiration. Our results demonstrate that N fertilization promoted ecosystem C fluxes, but that excessive N did not enhance the C fixation rate of the agroecosystem. These findings can inform global C-cycling models aimed at predicting C balances under N fertilization.