Developing a Critical Nitrogen Concentration Dilution Model and Diagnosing Nitrogen Nutrition of Wheat

The objective of this study was to verify the applicability of the critical nitrogen concentration dilution curve of wheat under different nitrogen application rates in the field and discuss the feasibility of using the nitrogen nutrition index (NNI) to optimize nitrogen fertilizer application. Experiments with different wheat varieties were carried out in Xinxiang (Henan Province, China) for two consecutive years, using four nitrogen application rates: 0, 165, 247.5, and 288.75 kg ha−1. Values of aboveground dry matter (t ha−1) and nitrogen concentration (%) were measured at each sampling date to develop the critical nitrogen concentration (Nc) equation. The relationship between Nc and aboveground dry matter (DM) was respectively described by a power function for 2018 and 2019 (Nc = 5.25DM −0.47, R2 = 0.931; and Nc = 5.30 DM −0.47, R2 = 0.924). Differences in dilution model parameters between years indicated that the nitrogen accumulation capacity in 2019 was slightly higher than that in 2018 and the slope of the Nc dilution curve was the same under the same aboveground biomass. Based on the Nc dilution model, the NNI model was established, which can be used to objectively and quantitatively diagnose nitrogen status. The results of nitrogen fertilizer regulation based on Nc and NNI were consistent with results of the nitrogen application rate based on yield indicators. The best nitrogen level was the 247.5 kg ha−1. Since Nc has reasonable biological significance, the model was accurate, simple, and with clear biological significance. It can be directly used in assessing nitrogen demand of crops and also in complex models of crop nitrogen dynamics. This will provide new ideas for timely and accurate fertilization.