Construction and verification of critical nitrogen dilution curve for spring maize varieties with different nitrogen efficiencies

This study investigated the effects of varying nitrogen application rates on spring maize to establish a critical nitrogen dilution curve based on variations in aboveground dry matter weight and plant nitrogen concentration at different growth stages. The objective is to provide a theoretical basis for the sustainable development and rational application of nitrogen fertilizers for spring maize in central Shanxi Province, China. Field experiments with the maize varieties ‘Zhengdan958’ (ZD958) and ‘Dafeng26’ (DF26) were conducted at the Dongyang Experiment Base of Shanxi Agricultural University from 2014 to 2016. Four nitrogen application treatments were tested: 0 kg(N)∙hm⁻² (N0), 120 kg(N)∙hm⁻² (N120), 240 kg(N)∙hm⁻² (N240), and 360 kg(N)∙hm⁻² (N360). Plant samples were collected at the jointing (V6), tasseling (VT), filling (R2), and maturity (R6) stages. The aboveground dry matter weight, yield, and plant nitrogen concentration at each growth stage were analyzed for both spring maize varieties under different nitrogen application rates. The critical nitrogen concentration dilution curve models for the two spring maize varieties were established and verified based on the aboveground dry matter weight and plant nitrogen concentration at various growth stages. Results indicated that ZD958 exhibited a higher nitrogen utilization rate than DF26. For both varieties, aboveground dry matter weight and yield enhanced with increasing nitrogen application rates at each growth stage. However, no significant difference in aboveground dry matter weight was observed between the N240 and N360 treatments, with the maximum grain yield being achieved under the N240 treatment. Under the conditions of suitable nitrogen application, plant nitrogen concentration increased with increasing nitrogen application, and showed a power exponential relationship with the growth period of the aboveground dry matter weight. Based on the relationship between aboveground dry matter weight (M_d) and plant nitrogen concentration (C_N) of spring maize, the critical nitrogen concentration dilution curve models were established as follows: for ZD958, C_N = 30.457 M_d^−0.292, and for DF26, C_N = 33.249 M_d^−0.333. Compared to the model parameters of DF26, parameter a of ZD958 decreased by 8.40%, and parameter b decreased by 12.31%. The model showed a linear correlation between the fitted and actual plant nitrogen concentrations, with root mean square errors (RMSE) of 1.71 g∙kg⁻¹ and 1.54 g∙kg⁻¹, and standard root mean square errors (n-RMSE) of 9.25% and 8.27%, respectively. The nitrogen nutrition index calculated from the critical nitrogen concentration curves increased with nitrogen application within the same growth stage, initially increasing and then decreasing with the growth period. There was a significant linear correlation between the nitrogen nutrition index and relative aboveground dry matter weight at each growth stage, while the relationship between the nitrogen nutrition index and relative yield followed a quadratic curve. These results suggest that the nitrogen nutrition index can be used to determine the nitrogen nutritional status of spring maize. In conclusion, the established critical nitrogen concentration dilution curve models and nitrogen nutrition indexes for the two spring maize varieties can be used to diagnose and evaluate the nutritional status of spring maize during its growth stages. Based on the relationship between nitrogen application rate and yield, it is recommended that the nitrogen application rate for ZD958 should be between 189.16 and 224.08 kg(N)·hm⁻², and for DF26 between 199.72 and 214.67 kg(N)·hm⁻².