Change in Nitrogen Requirement with Increasing Grain Yield for Winter Wheat in Northern China.

Overestimates of nitrogen (N) requirements have led to excessive N application and serious environmental pollution in intensive agricultural systems, especially in rapidly developing countries. A database comprising 1410 measurements was developed from 2000 to 2011 using 89 on-farm and station experiments conducted in five key winter wheat (Triticum aestivum L.) domains in Northern China. The database was created to evaluate the relationship between aboveground N uptake and grain yield with different N treatments and to quantify N requirements per Mg grain at different wheat grain yield levels. Across all sites, winter wheat grain yield ranged from 1.6 to 11.8 Mg ha^-1. The N requirement per Mg grain yield (N_req.) increased with N supply and the average N_req. was 24.3 kg for the optimal N fertilizer treatment. For a yield range of <4.5 to 6.0-7.5 Mg ha^-1, N_req. decreased from 27.1 to 24.5 kg due to increasing harvest index (HI; from 0.39 to 0.46) and decreasing grain N concentration (from 2.41% to 2.21%). For a yield range of 6.0-7.5 to 9.0-10.5 Mg ha^-1, N _req. decreased from 24.5 to 22.8 kg due to decreasing grain N concentration (from 2.21% to 2.00%). For a yield range of 9.0-10.5 to >10.5 Mg ha^-1, N_req. changed little due to stability in grain N concentrations and HI. In conclusion, the N requirement of a crop was affected by both the level of N supply and grain yield.