Effects of water-nitrogen interactions on the fate of nitrogen fertilizer in a wheat-soil system

In recent decades, continuous increase of N fertilizer inputs made significant contributions to the wheat pro-duction increase in the Huang-Huai-Hai Plain, the largest wheat producing region of China. However, gradually the use efficiency of N fertilizer decreased, and the increasing N rate led to a series of environment problems. A better quantitative understanding of the fates of basal N and topdressing N under water-nitrogen interactions in a wheat-soil system is essential to increase yield and reduce environmental impacts. We conducted two-year wheat experiments combining field plots and micro-plots with the l5N-labeled method under two irrigation regimens (rainfall, irrigation in jointing and anthesis) and three N rates (0, 180, and 270 kg ha(-1)). The results showed that 54.9%-70.4% of total wheat N accumulation was from soil native N, while 29.6%-45.1% was from fertilizer N. Irrigation at joint and anthesis has increased wheat N accumulation more from fertilizer N than from soil native N. Compared with rainfed condition, irrigation significantly increased plant NNI at booting and anthesis. The contribution of pre-anthesis N translocation to grain N was 80.6%-86.0%, and increased with increasing total N rate, but decreased with irrigation. The fertilizer N recovery rate increased as N rate and irrigation increased, and more N recovery occurred during jointing to anthesis period (13.1%-31.3%) than anthesis to maturity period (5.7%-10.8%) and sowing to jointing period (8.4%-8.9%). Residual fertilizer N in soil accounted for 24.5%-38.6% and decreased with increasing total N rate and irrigation. 23.6%-32.7% of fertilizer N was lost into the environment, and it decreased with increasing irrigation, but wasn't affected by total N rate. The recovery and residual for topdressing N were higher than those for basal N, whereas loss was lower. Moreover, the loss of basal N mainly occurred before jointing, and the loss of topdressing N mainly occurred from jointing to anthesis and contributed more to total N loss. These results indicated that the current wheat-soil system in Huang-Huai-Hai Plain has substantial potential to coordinate the synchronization of N demand and N supply, and finally reduce N loss. Also, this would provide critical insights to construct general crop N management models for precise N management.