Increased nitrogen use efficiencies as a key mitigation alternative to reduce nitrate leaching in North China plain

The Northern China Plain (NCP) produces over 20% of the national grain production. Best management practices (BMP) for intensive irrigated cropping systems of the NCP are based on large nitrogen (N) applications without accounting for N budgets. There are concerns that non-scientific based BMPs may be impacting underground water resources. We conducted the first study in this region, located at the Luancheng Experimental Research Station that measured the effects of N fertilizer rates on nitrate-nitrogen (NO3-N) leaching losses. From October 1, 2001 to September 30, 2004, we used a water balance approach with a neutron probe, weighing lysimeter, and suction cups located at 1.8 m depths on a winter wheat (Triticum aestivum L.)-corn (Zea mays L.) rotation to monitor NO3-N leaching. Residual soil NO3-N, yields, and N uptake by aboveground biomass were also measured. Corn received two surface broadcast applications every year of 50, 100, 150 and 200 kg urea-N ha(-1) for the N-200, N-400, N-600, and N-800 treatments, respectively. The first broadcast application was at seeding and the second at tassel. Similarly, winter wheat received two surface broadcast applications, initially as a pre-plant and a second application at the jointing stage of growth in spring. We monitored NO3-N leaching losses for the N-200, N-400, and N-800 treatments. Average NO3-N leaching losses during wheat-corn season were 6, 58, and 149 kg NO3-N ha(-1) year(-1) for the 200, 400, and 800 kg N ha(-1) year(-1) treatments, respectively. The NO3-N leaching increased with N applications (P < 0.05) and were in agreement with the NO3-N concentrations of 12, 74, and 223 mg NO3-N L-1 for soil water at 1.8 m depths for the 200, 400, and 800 kg N ha(-1) year(-1) treatments, respectively. Higher than needed N fertilizer applications increased the NO3-N leaching losses and reduced the N use efficiency (NUE) without yield increases. We propose that there is a need for a new scientifically based BMP approach for the NCP based on N budgets that credits soil NO3-N before planting, N mineralization from soil organic matter, and other potential N sources as a key mitigation alternative to increase NUE and reduce NO3-N leaching in this region.