Effects of Irrigation Regime and Nitrogen Rate on Yield, Apparent Water Use Efficiency, and Net Return of Winter Wheat Under Center-Pivot Fertigation

To identify an irrigation and nitrogen management strategy balancing winter wheat yield, resource-use efficiency, and economic return under center-pivot fertigation, a two-season field experiment was conducted in Yangling, Shaanxi Province. Four stage-specific irrigation regimes (30, 60, 90, and 120 mm) and four nitrogen rates (0, 150, 225, and 300 kg N ha−1) were evaluated. The 90 mm irrigation regime combined with 225 kg N ha−1 achieved the highest two-season mean grain yield, apparent water use efficiency calculated from the 0–60 cm soil water balance, and net return, reaching 8200.06 kg ha−1, 23.28 kg ha−1 mm−1, and 7593.76 CNY ha−1, respectively. The highest partial factor productivity of applied nitrogen (47.21 kg kg−1) occurred with 90 mm irrigation and 150 kg N ha−1. Increasing nitrogen from 225 to 300 kg N ha−1 under 90 mm irrigation reduced mean grain yield and net return by 603.55 kg ha−1 and 1998.00 CNY ha−1, respectively. Overall, 90 mm stage-specific irrigation combined with 225 kg N ha−1 was a promising strategy among the tested combinations. Response surface analysis indicated a predicted net-return optimum at approximately 209 kg N ha−1 under the fixed 90 mm irrigation regime.