Soil nitrate accumulation, leaching and crop nitrogen use as influenced by fertilization and irrigation in an intensive wheat-maize double cropping system in the North China Plain

There is a growing concern about excessive nitrogen (N) and water use in agricultural systems in North China due to the reduced resource use efficiency and increased groundwater pollution. A two-year experiment with two soil moisture by four N treatments was conducted to investigate the effects of N application rates and soil moisture on soil N dynamics, crop yield, N uptake and use efficiency in an intensive wheat-maize double cropping system (wheat-maize rotation) in the North China Plain. Under the experimental conditions, crop yield of both wheat and maize did not increase significantly at N rates above 200 kg N ha(-1). Nitrogen application rates affected little on ammonium-N (NH4-N) content in the 0-100 cm soil profiles. Excess nitrate-N (NO3-N), ranging from 221 kg N ha(-1) to 620 kg N ha(-1), accumulated in the 0-100 cm soil profile at the end of second rotation in the treatments with N rates of 200 kg N ha(-1) and 300 kg N ha(-1). In general, maize crop has higher N use efficiency than wheat crop. Higher NO3-N leaching occurred in maize season than in wheat season due to more water leakage caused by the concentrated summer rainfall. The results of this study indicate that the optimum N rate may be much lower than that used in many areas in the North China Plain given the high level of N already in the soil, and there is great potential for reducing N inputs to increase N use efficiency and to mitigate N leaching into the groundwater. Avoiding excess water leakage through controlled irrigation and matching N application to crop N demand is the key to reduce NO3-N leaching and maintain crop yield. Such management requires knowledge of crop water and N demand and soil N dynamics as they change with variable climate temporally and spatially. Simulation modeling can capture those interactions and is considered as a powerful tool to assist in the future optimization of N and irrigation managements.