A Modified Critical Nitrogen Dilution Curve for Winter Wheat to Diagnose Nitrogen Status Under Different Nitrogen and...

The accuracy of nitrogen (N) diagnosis is essential to improve N use efficiency. The standard critical N concentration (standard N_c) dilution curves, an expression of the dynamics of N uptake and dry matter accumulation in plants, are widely used to diagnose the N status of crops. Several standard N_c dilution curves were proposed and validated for several crops, based on experiments involving different N fertilizer treatments. However, standard N_c dilution curves are affected by crop water status, e.g., resulting from differences in irrigation management. This paper aimed at developing a N diagnostic model under the coupling effect of irrigation and fertilizer managements. For this purpose, N_c dilution curves were developed under different irrigation rates. Additionally, plant water content (PWC), leaf water content (LWC), leaf area index (LAI), equivalent water thickness (EWT), and leaf area duration (LAD) were introduced into the model, to construct a modified N_c (mN_c) dilution curve. The mN_c dilution curves were designed using the principle of hierarchical linear model (HLM), introducing aboveground dry biomass (AGB) as the first layer of information, whereas the second layer of information included the different agronomic variables (PWC, LWC, LAI, EWT, and LAD). The results showed that parameters "a" and "b" of the standard N_c dilution curves ranged from 5.17 to 6.52 and -0.69 to -0.38 respectively. Parameter "a" was easily affected by different management conditions. The performance of standard N_c dilution models obtained by the cross-validation method was worse than that of mN_c dilution models. The N_c dilution curve based on 4 years of data was described by the negative power equation N_c = 5.05 × AGB^-0.47, with R ² and nRMSE of 0.63 and 0.21, respectively. The mN_c dilution curve considers different treatments and was represented by the equation mN_c = a×AGB^- , where a = 2.09 × PWC + 3.24, b = -0.02 × LAI + 0.51, with R ² and nRMSE of 0.79 and 0.13, respectively. For winter wheat, C₃ crop, there would be a few problems in using standard N_c dilution methods to guide field management, however, this study provides a reliable method for constructing mN_c dilution curves under different water and N fertilizer management. Due to the significant differences in hereditary, CO₂ fixation efficiency and N metabolism pathways for C₃ and C₄ crops, the construction of mN_c dilution curve suitable for different N response mechanisms will be conducive to the sustainable N management in crop plants.