Optimization of irrigation and nitrogen fertilizer management for spring maize in northwestern China using RZWQM2

Irrigation and nitrogen fertilizer management (INM) are essential for maize production. In northwest China, the conventional INM, in which irrigation of about 90-120 mm is carried out four or five times with large nitrogen (N) fertilizer inputs (approximately 450 kg N ha(-1)), generally result in low water use efficiency (WUE) and N use efficiency (NUE) with a series of environmental pollution problems. The main objective of this study was to optimize the INM for spring maize using the Root Zone Water Quality Model 2 (RZWQM2). The RZWQM2 was first calibrated and verified based on a two-year experiment. The NRMSEs (normalized root mean square errors) for the simulated grain yield, aboveground biomass, plant N uptake and grain N uptake were 7.02%, 6.75%, 10.73% and 12.40%, respectively, in the calibration period; were 9.73%, 7.96%, 12.00% and 15.34%, respectively, in the validation period. The NRMSEs for the simulated soil water content and soil nitrate N content at different depths ranged from 8.29% to 24.52% and from 13.34% to 32.83% across the calibration and validation period, respectively. The calibrated model was used to investigate grain yield, biomass, plant-level WUE (WUEp) and plant-level NUE (NUEp) under different transpiration and plant N uptake characteristics of maize plants. Simulation results indicated that the maize plants with high WUEp generally have high NUEp, and reducing transpiration is an effective way to improve either WUEp or NUEp. Furthermore, the calibrated model was used to evaluate different INM options including N application rates, split N strategies and irrigation strategies. Based on simulation results, split N application is not necessary if water drainage is effectively controlled. The optimal N fertilizer rate is 200 kg ha(-1) N and is encouraged to be applied once as a base fertilizer. The irrigation practice, in which irrigation is adopted to refill the primary root zone (PRZ, 0-60 cm) to available water capacity (AWC) when available water in the PRZ decreases to 15% AWC, achieved the highest WUE (2.23 kg m(-3)) among all alternative irrigation practice options with a high grain yield (12.6 t ha(-1)) and so is recommended. Compared with conventional INM, the optimal INM decreased irrigation amount by 14.7%, water drainage by 53.7% and N leaching by 80.5% but increased WUE by 8.9% with a comparable grain yield.