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

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(−b), 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, C(3) 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, CO(2) fixation efficiency and N metabolism pathways for C(3) and C(4) 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.