Simulation of the threshold of maize nitrogen application using a DNDC model in salinized irrigation areas

Abstract: This study aims to determine the suitable nitrogen application rate for agricultural production and environmental protection in salinized irrigation areas. A two-year field experiment was carried out in the Hetao Irrigation District of Inner Mongolia in Western China. A Denitrification-Decomposition (DNDC) model was selected to simulate the key factors, including the amount of nitrate leaching loss, the amount of plant nitrogen uptake, and the threshold of maize nitrogen application rate. The results showed that: 1) The DNDC model accurately simulated the maize yield and nitrogen utilization. A better agreement between the measured and simulated data was achieved on the maize yield, leaf area index (LAI) and soil nitrate-nitrogen accumulation in the 0-20 cm soil layer, where the model Nash-Sutcliffe efficiency (ENS) and the coefficient of determination (R2) were all greater than 0.75, the normalized root mean square error (NRMSE) was 9.26%-9.57%, 13.49%-17.51%, and 19.84%-21.48%, respectively. 2) A sensitivity analysis of the model parameters showed that the nitrogen application rate (the sensitivity index of 1.92 and -0.89) and topdressing times (the sensitivity index of 0.11, and -0.85) presented significant effects on the amount of nitrate leaching loss and plant nitrogen uptake. However, the tillage depth (the sensitivity index of 0.03, and 0.09) and irrigation amount (the sensitivity index of -0.02, and -0.01) behaved little effect on the amount of nitrate leaching loss and plant nitrogen uptake. Whereas, the excessive application of nitrogen rate cannot promote the amount of plant nitrogen uptake and yield, but lead to the amount of nitrate leaching and even environmental pollution. 3) The amount of plant nitrogen uptake and maize yield increased firstly and then gradually stabilized, with the increase of nitrogen application rate. In addition, the amount of plant nitrogen uptake during the growth period increased by 167.18% and 31.27%, when the number of topdressing was three times, compared with one and two times. 4) Once the number of topdressing was the same, the nitrate leaching loss increased with the increase of nitrogen application rate. However, the leaching loss of nitrate decreased gradually with the increase of topdressing times, when the amount of nitrogen rate was constant. The nitrate leaching loss in the growing season decreased by 41.96% and 59.75%, when the number of topdressing was two or three times, compared with the single. As such, the optimal fertilization was 165.50-200 kg/hm2 nitrogen application rate, and top fertilization at the jointing, tasseling, and filling stages, considering the yield, the nitrate leaching loss, and the nitrogen rate applied with the maximum amount of plant nitrogen uptake. The finding can provide technical support to reduce the groundwater pollution and resource wastes in Hetao Irrigation District.