Objectives  Film mulching days affect nutrient transport and N use in cotton fields. Here, we used the root zone water quality model-version 2 (RZWQM2) to simulate the N use efficiency of cotton and N transport in the soil. We aim to provide a theoretical base for choosing suitable mulching days in oasis cotton fields in Xinjiang.

  Methods  Field experiments were conducted in Xinjiang from 2017 to 2018; the mulching day treatments were set up as 40, 55, 70, 85, and 100 days across the growth stages of cotton. From seedling to boll opening stage, we measured the soil water and nitrate content in 0−80 cm depth, N absorption amount of cotton was measured at maturity. The measured soil water storage, nitrate-nitrogen content, and plant N uptake across the growth stages of cotton in the film mulching treatments were used to calibrate the RZWQM2 model. The values recorded for the indexes mentioned above under different mulching day treatments were used to verify the reliability of the calibrated model. Finally, the model calculated the change in cotton N uptake and N use efficiency across the mulching days during 55−105 days of cotton growth.

  Results  After calibration with the measured values under mulching treatment across the growing period of cotton, the simulated soil water storage and NO₃^–-N content showed satisfactory agreement with the measured values. The root means square error (RMSE) between the measured and simulated values ranged from 1.14 to 2.87 mm and 2.05 to 17.28 mg/kg; the mean square errors (MRE) ranged from 6.61%–20.08% and 0.06%–0.57%, respectively. The simulated and measured values of cotton N uptake were in good agreement, with the peak values appearing at 94–96 days and 97–99 days, respectively. The total N loss (ammonia volatilization + denitrification + leakage) decreased with increasing mulching time. The lowest N leakage was observed at 94–99 days mulching. Agronomic and physiological utilization rates responded well to the film mulching time. Simulating the N uptake and use efficiencies of cotton with the elongation of mulching days by the model revealed gaps in the optimum mulching days due to climate impact in 2019 and 2020.

  Conclusions  After calibration and validation, the RZWQM2 model successfully evaluated the mulching days for cotton N use efficiency and N uptake. The RMSE and MRE between the simulated and measured values were within the acceptable ranges. The change in N uptake and N use efficiency of cotton with mulching days fully reflected the climate differences, indicating the objectivity and reliability of the model. The optimum mulching time of 94–99 days in Xinjiang was beneficial to reduce the risk of nitrogen leaching loss, improve the nitrogen use efficiency of cotton plants.