Optimized water-nitrogen synergy enhances winter wheat yield and nitrogen use efficiency in clay-loam fluvo-aquic soils of southeastern Henan, China

To reduce yield variability and improve nitrogen use efficiency in winter wheat grown on clay loam fluvo-aquic soils in southeastern Henan—where uneven precipitation and excessive N inputs are common—we conducted a two-year split-plot field experiment. Main plots included two water regimes: rainfed (W0) and supplemental irrigation at both jointing and anthesis (W2; 75 mm per event). Subplots included five N rates: 0, 180, 240, 300, and 360 kg hm⁻² N (N0-N4). We assessed the combined effects of water and N on dry matter accumulation, N utilization, and grain yield. W2 significantly increased post-anthesis dry matter assimilation; compared with W0, dry matter accumulation from anthesis to filling increased by 32.2%, and total dry matter accumulation increased by 28.1%. In parallel, N uptake efficiency and the partial factor productivity of N increased by 26.7% and 25.5%, respectively. Water-N interactions synergistically regulated pre-anthesis N translocation and post-anthesis assimilate partitioning, thereby improving all three yield components. Under the optimal combination (W2N2), spike number, grains per spike, and thousand-grain weight reached 615.0 × 10⁴ hm⁻², 43.9 grains spike⁻¹, and 45.6 g, respectively. Relative to the traditional high-N practice (W2N4), grains per spike and thousand-grain weight increased by 2.0% and 2.3%, respectively; relative to rainfed with reduced N (W0N2), spike number and grains per spike increased by 11.1% and 34.6%, respectively. These results indicate that W2 increased yield mainly by increasing spike number and grains per spike, whereas reducing N to N2 optimized grains per spike while maintaining a relatively high thousand-grain weight on the basis of stable spike number, forming a synergistic yield strategy characterized by “spike number as the foundation, grains per spike as the driver, and thousand-grain weight stabilization.” This strategy produced the highest yield (9331.8 kg hm⁻²) and increased the partial factor productivity of N and agronomic N use efficiency by 61.2% and 67.9%, respectively, compared with 360 kg hm⁻² N. Regarding soil environmental effects, supplemental irrigation altered the vertical distribution of nitrate-N, decreasing residual nitrate by 23.4% in the 0-40 cm layer but increasing it by 32.2% in the 40-100 cm layer, suggesting that irrigation optimization alone may promote downward N movement. However, combining W2 with N reduction to N2 reduced cumulative nitrate-N in the 0-100 cm profile by 26.9%, thereby mitigating leaching risk. Overall, applying 75 mm supplemental irrigation at jointing and anthesis together with 240 kg hm⁻² N provides an effective water-N management strategy that synchronously improves yield and N use efficiency for winter wheat production in the clay loam fluvo-aquic soil region of southeastern Henan.