Single spring irrigation with optimized nitrogen sustains winter wheat yield and improves water productivity and profitability in the North China Plain

Optimizing irrigation and nitrogen (N) management is critical for sustainable winter wheat production in the North China Plain (NCP). To clarify the optimal N rate under a single spring irrigation regime, a split-plot field experiment (2020–2023) compared two irrigation schedules—W2: 75 mm at the spring 3-leaf stage and again at anthesis; W1: a single 75 mm irrigation at the spring 4-leaf stage—crossed with four N rates (N1–N4: 180, 240, 300, 360 kg N ha−1). Relative to W2, W1 reduced evapotranspiration, increased soil water consumption, and redistributed water use toward the jointing–anthesis stage. However, W1 incurred a 3.3–11.6% yield penalty compared with W2. Under W1, applying 300 kg N ha−1 (N3) maximized spike number and flag-leaf area, lowered canopy temperature, maintained flag-leaf photosynthetic capacity, increased dry matter accumulation and harvest index, and consequently produced the highest grain yield (8351.5 kg ha−1). The W1N3 combination achieved the greatest crop water productivity (2.1 kg m−3), irrigation water productivity (11.1 kg m−3), agronomic N efficiency (11.2 kg kg−1), net income, and benefit-cost ratio. Quadratic response modeling indicated that, under W1, an N range of ≈279–298 kg ha−1 attains 95–100% of the maximum yield and net income. These results provide actionable guidance for water-saving wheat cultivation in the NCP: single spring irrigation at the 4-leaf stage paired with ∼280–300 kg N ha−1 can largely offset yield penalties while improving resource-use efficiency and farm profitability.