Regional evaluation of critical nitrogen concentrations in winter wheat production of the north China plain

Investigating critical nitrogen concentration (CNC) in grain and straw provides insights into N nutrition, and can serve as a guide to improved agricultural practice. This regional study evaluated the relationship between N fertilization rate and grain yield, N concentration, potential N loss, and determined critical grain and straw nitrogen concentrations (CGNC and CSNC) for winter wheat (Triticum aestivum L.) production in China. At the economically optimum nitrogen rate (EONR), grain N concentration was similar to the maximum value calculated using a linear plus plateau model, while straw N concentration was significantly less than the relevant maximum value. Soil nitrate N content after harvest and apparent N loss for maximum straw N concentration increased by 19 and 9 kg N ha^-1 compared to values at the EONR. Based on nine field experiments, CGNC and CSNC corresponding to optimal N rate were established to be 21.9 g kg^-1 (20.8-23.0 g kg^-1) and 6.8 g kg^-1 (6.5-7.1 g kg^-1), respectively. An evaluation of CGNC and CSNC across 111 on-farm sites indicated that while many sites had grain and straw N concentrations falling within the CGNC and CSNC, a substantial portion of the sites had grain and straw N concentrations falling outside of the CGNC and CSNC or falling within the critical ranges when N supply was deficient (0 N control) or excess (at farmer's N practice). This region-wide study provided evidence for the usefulness of CSNC, and particularly CGNC, as indicators of N deficiencies in wheat production; however, neither indicator provided information about excess N fertilization.