Overestimates of N requirements have led to excessive N application and serious environmental pollution in intensively managed agricultural systems. A database comprising 1395 measurements was developed from 2000 to 2011 using 88 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 grain yield levels. Across all sites, wheat grain yield ranged from 1.6 to 11.8 Mg ha -1. The nitrogen requirement per megagram grain yield (N req.) increased with N supply from 20.8 kg under the treatment without N fertilizer to 25.7 kg under the excess N treatment. For the optimal N fertilizer treatment, the average N req. was 24.3 kg and it declined with increasing grain yield. For the yield ranges between 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 amount of N supplied and the grain yield.
