Soil Fertility-Dependent Critical Nitrogen Dilution Curves in Winter Wheat: Evaluating Cumulative Evapotranspiration as an Auxiliary Growth Indicator

Abstract

Critical nitrogen (N) dilution curves are widely used to diagnose crop N status, but their parameters may be affected by indigenous soil N supply, and conventional curves require repeated biomass sampling. This study developed fertility-specific critical N concentration (N_c) curves for irrigated winter wheat using aboveground dry matter (DM) and cumulative evapotranspiration (ET), and evaluated ET as an auxiliary growth indicator for N diagnosis. A 2 year field experiment was conducted under medium- and low-fertility conditions with seven N rates, ranging from 0 to 270 kg N ha⁻¹. DM-based and ET-based N_c curves were fitted separately by fertility level. The nitrogen nutrition index (NNI), accumulated N deficit (N_and), and their relationships with relative grain yield were analysed using method-specific plateau-linear thresholds. Soil fertility clearly altered crop growth, yield response, and N dilution. DM-based curves were N_c = 5.53 × DM^−0.47 for medium fertility and N_c = 4.43 × DM^−0.45 for low fertility, while ET-based curves were N_c = 458.00 × ET^−0.98 and N_c = 205.94 × ET^−0.86, respectively. Cumulative ET was strongly associated with DM and produced NNI and N_and dynamics broadly consistent with DM-based diagnosis. However, ET-based NNI thresholds, ranging from 0.826 to 0.847, were lower than DM-based thresholds, ranging from 0.926 to 0.966, indicating that ET-derived diagnosis requires method-specific calibration rather than direct use of the conventional NNI = 1 criterion. Overall, these findings highlight the importance of accounting for soil fertility in critical N dilution modelling and demonstrate the potential of cumulative ET as a complementary growth-related indicator for developing calibrated in-season N diagnostic strategies in irrigated winter wheat.