Optimization of Water–Nitrogen Management for Winter Wheat in the North China Plain: Balancing Yield and Quality

The North China Plain is a major wheat-producing region in China where appropriate irrigation and nitrogen management are essential for coordinating grain yield, grain nutritional quality, and resource-use efficiency. We hypothesized that moderate water and nitrogen inputs would achieve a better balance among these objectives than excessive or insufficient inputs. The objective of this study was to quantify the effects of irrigation and nitrogen application on wheat growth, grain yield, selected grain nutritional quality indicators, and water–nitrogen-use efficiency, and to identify a management regime that balances these objectives using the entropy-weighted TOPSIS method. To test this hypothesis, a two-year field experiment was conducted with three irrigation levels (350, 275, and 200 mm) and four nitrogen application rates (0, 120, 240, and 360 kg ha−1). Wheat growth, photosynthetic traits, dry-matter accumulation, grain yield, selected grain nutritional quality indicators, irrigation water-use efficiency (IWUE), and nitrogen partial factor productivity (NPFP) were evaluated. The results showed that high irrigation combined with high nitrogen application promoted plant height, chlorophyll content, photosynthetic activity, and dry-matter accumulation. However, grain yield and resource-use efficiency did not increase continuously with increasing water or nitrogen inputs. The moderate-irrigation and moderate-nitrogen treatment (IMNW) produced the highest grain yield in both growing seasons and achieved the highest comprehensive evaluation score in the entropy-weighted TOPSIS assessment. Regression analysis identified overlapping suitable input ranges of 295–316 mm for annual irrigation and 210–278 kg ha−1 for nitrogen application. Therefore, under the experimental conditions of the North China Plain, moderate water and nitrogen inputs provided a better balance among grain yield, selected grain nutritional quality indicators, and resource-use efficiency.