To address the issues of excessive nitrogen application and water scarcity in the irrigation area of northern Henan, the aim of this study was to explore the regulating effect of annual water and nitrogen optimization (reducing water and nitrogen by 25%) on crop growth and nitrogen loss in the wheat-maize rotation system, so as to provide a theoretical basis and technical support for the sustainable agricultural practices in the wheat-maize rotation system of this region.

A five-year (2018-2023) field experiment was conducted in Huaxian County, Anyang City of Henan province, and three water and nitrogen management treatments were set up: local traditional water and nitrogen treatment (TP, annual nitrogen application of 552.0 kg·hm^-2 combined with irrigation of 270.0 mm), nitrogen factor optimization treatment (OF, annual nitrogen application of 414.0 kg·hm^-2 combined with irrigation of 270.0 mm), and water and nitrogen factor optimization treatment (OWF, annual nitrogen application of 414.0 kg·hm^-2 combined with irrigation of 202.5 mm). The effects of annual water and nitrogen optimization on the yield, biomass, nitrogen accumulation, nitrogen harvest index (NHI), nitrogen partial factor productivity (NPFP), and nitrogen uptake and physiological efficiency (NUPE), soil moisture and nitrate nitrogen content, nitrogen leaching and runoff, and annual nitrogen balance of the wheat-maize rotation system were studied.

Compared with TP and OF treatments, OWF treatment had no significant effect on the annual average yield, aboveground biomass, grain nitrogen accumulation, and aboveground nitrogen accumulation of wheat and maize, but significantly increased NPFP by 31.4% and 0.9%, NUPE by 28.5% and 0.1%, respectively. There was a significant positive correlation among yield, biomass, grain nitrogen accumulation, and aboveground nitrogen accumulation. The annual optimization of water and nitrogen management significantly reduced soil moisture content and NO₃^--N content at different growth stages. Compared with TP and OF treatments, OWF treatment resulted in a significant reduction in soil moisture content by 6.6% and 9.6%, respectively, and a significant decrease in NO₃^--N content by 46.1% and 37.9%, respectively. The wheat season and maize season accounted for 2.9% and 97.1% of the annual average nitrogen leaching loss, respectively. The annual average nitrogen leaching losses under TP, OF, and OWF treatments were 52.9, 45.8, and 39.6 kg·hm^-2, respectively. Compared with TP and OF treatments, OWF treatment significantly reduced the annual average NO₃^--N leaching loss by 25.2% and 13.5%, respectively, and decreased NO₃^--N surface runoff by 32.0% and 18.1%, respectively. The annual average nitrogen surplus in the irrigation area of northern Henan was 60.6 kg·hm^-2. Compared with TP treatment, the nitrogen surplus under OWF treatment was reduced by 81.9%, while compared with OF treatment, it was increased by 20.8%.

Annual water and nitrogen optimization could maintain stable crop yields in the wheat-maize rotation system while enhancing the efficient utilization of water and nitrogen resources. This approach significantly reduced soil nitrate nitrogen (NO₃^--N) content across the 0-100 cm soil layers, mitigated NO₃^--N leaching, promoted soil nitrogen balance, and reduced the risk of non-point source pollution in the irrigation area of northern Henan. These results demonstrated that water and nitrogen optimization served as a green and efficient management measure for the wheat-maize rotation system in this region.