Change in Nitrogen Requirement with Increasing Grain Yield for Winter Wheat

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 (Nreq.) 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 Nreq. was 24.3 kg and it declined with increasing grain yield. For the yield ranges between <4.5 and 6.0 to 7.5 Mg ha−1, Nreq. decreased from 27.1 to 24.5 kg due to increasing harvest index (HI; from 0.39–0.46) and decreasing grain N concentration (from 2.41 to 2.21%). For the yield ranges between 6.0 to 7.5 and 9.0 to 10.5 Mg ha−1, Nreq. decreased from 24.5 to 22.7 kg due to decreasing grain N concentration (from 2.21 to 2.00%). For the yield ranges between 9.0 to 10.5 and >10.5 Mg ha−1, Nreq. 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.