Effects of Irrigation Lower Limit and Nitrogen Rate on Productivity, Resource Use Efficiency, and Economic Benefits of Winter Rapeseed in Semi-Arid Conditions

Integrated water and nitrogen management plays a crucial role in the sustainable intensification of rapeseed production, particularly in water-limited regions. This two-year field study (2022–2024) evaluated the interactive effects of three irrigation lower limits—W1 (90% of field capacity, [FC]), W2 (70% FC), and W3 (50% FC)—and four nitrogen rates (0, 80, 160, and 240 kg N ha−1; representing N0, N1, N2, N3, and N4) on winter rapeseed growth, yield, resource use efficiency, and economic performance under semi-arid conditions. Both irrigation and nitrogen significantly influenced plant growth, photosynthetic performance, biomass accumulation, and yield formation, with pronounced interactive effects observed across most measured parameters. The W1N2 treatment achieved optimal performance, producing seed yields of 5131 and 3220 kg ha−1 with superior nitrogen use efficiency. Overall, N1, N2, and N3 increased yield by 38.12%, 79.26%, and 84.85%, respectively, relative to N0. Compared with W3N0, W1N2 improved yield by 178%, water use efficiency by 131%, and irrigation water use efficiency by 110%. Relative to W1N3, W1N2 increased nitrogen agronomic efficiency, physiological efficiency, recovery efficiency, and partial factor productivity by 40.5%, 7.4%, 30.4%, and 45.2%, respectively, while reducing nitrate nitrogen residue by 12%. Entropy-TOPSIS analysis identified W1N2 as the top-ranked treatment, indicating that optimized irrigation and nitrogen management offer a sustainable strategy to maximize rapeseed productivity, enhance resource-use efficiency, and improve economic returns under water-limited conditions. For practical application in semi-arid environments, the W1N2 treatment is recommended as the optimal management strategy for sustainable winter rapeseed intensification.