Abstract: To explore precision nitrogen application by using maize root index, we investigated the dynamics of maize root biomass and plant nitrogen concentration by means of crop critical nitrogen concentration model, and revealed the optimum amount of nitrogen application to maize under drip irrigation in Ningxia Yellow River Irrigation Area. There were six nitrogen application levels. The critical nitrogen concentration model of maize under drip irrigation based on root biomass was established by statistical analysis and modeling method. The nitrogen nutrition index (NNI) of roots was calculated to analyze the yield benefit of maize under drip irrigation across different treatments. Results showed that root biomass, root nitrogen accumulation, and yield of maize under drip irrigation were significantly affected by nitrogen application level. Root biomass of each treatment showed a gradually increasing trend but root nitrogen concentration showed a downward trend with maize growth. By combining the data of different nitrogen levels in the first two years, the critical nitrogen concentration dilution curve of maize based on drip irrigation of root dry matter was constructed as N_c=1.97W^-0.59 (R²=0.92). The model validation results with the test data in the third year showed that R² was 0.91, RMSE and NRMSE were 0.15 g·kg^-1 and 7.49%, respectively, indicating high accuracy of the model. Based on the NNI calculation and the analysis of yield benefit, the optimum nitrogen application rate of maize under drip irrigation in Ningxia Yellow River Irrigation Area was estimated to be 270-311 kg·hm^-2. Our results could provide theoretical guidance for precise nitrogen application of maize under drip irrigation in Ningxia.

Key words: maize, integrative water and fertilizer application, root biomass, nitrogen concentration, critical nitrogen dilution curve, nitrogen nutrition index.