Mitigating ammonia volatilization and increasing nitrogen use efficiency through appropriate nitrogen management under supplemental irrigation and rain–fed condition in winter wheat

Agricultural ammonia (NH3) emissions significantly reduce nitrogen (N) use efficiency (NUE) and adversely affect environmental quality. There is thus much interest in mitigating NH3 emissions through appropriate N fertilizer and water management in winter wheat. A two–year field study –was conducted to quantify the recovery of 15N–labelled N fertilizer and to investigate the NH3 flux, grain yield, yield–scaled NH3 emissions, NUEs and water use efficiency (WUE) among different N application rates (0, 120, 240, and 360 kg N ha–1, abbreviated as N0, N120, N240 and N360, respectively) under rain–fed and supplemental irrigation condition. The peak daily NH3 emissions reached 2–8 DAS after basal N fertilizer application, and the fluxes decreased to relatively low levels thereafter. Most NH3 volatilization (> 90%) occurred within one month after sowing. The maximum accumulative NH3 emission was 66 kg N ha–1 in N360, which was increased by 104%, 2–fold and 12–fold compared with N240, N120 and N0, respectively, averaged across water regimes and experimental years. The NH3 emission factor (%) of N360 was significantly increased by 43% and 77% compared with the other two N application rates (N120 and N240) under rain–fed and supplemental irrigation regimes, respectively. Water regimes did not have a significant effect on NH3 fluxes or NH3 emission factors (%). Grain yield, WUE and yield–scaled NH3 emissions generally increased with increasing N application rate, and the highest values were achieved under N120 or N240 regardless of the water regimes. All NUE traits were reduced, and the 15N residue in the soil profiles increased with increasing N application rates under both water regimes. The results suggested that high NH3 emissions were the major reason for inferior NUE, which was further confirmed by the 15N stable isotope. Overall, the medium N rate (120–240 kg N ha–1) is the optimal rate for achieving a high yield of 8 t ha–1, as further increases in N rates fail to produce significantly higher yield, and even increase NH3 emissions, lead to the deterioration of WUE and NUE, and cause serious environmental costs.