Straw incorporation surpasses mulching for wheat yield and water use efficiency under deficit irrigation on the North China Plain

In the water-scarce North China Plain (NCP), choosing between straw mulching (SM) for surface conservation and straw incorporation (SI) for soil amelioration is a critical agricultural dilemma. However, their effects on soil water dynamics and crop productivity under different irrigation levels remain unclear. We conducted a two-year field experiment comparing SI and SM under three irrigation levels in winter wheat: no irrigation (I0), 60 mm irrigation at jointing (I60), and 60 mm irrigation at both jointing and heading (I60 + 60). Soil water dynamics were monitored, field-scale evapotranspiration (ET) was estimated using a water-balance approach, and aboveground dry matter, yield components, and water use efficiency (WUE) were measured. SM maintained higher topsoil water content early in the season, whereas SI generally showed greater changes in soil water storage (DSWS) and higher mean daily ET. Despite weaker early-stage surface water conservation, SI increased aboveground dry matter and grain yield by 5.2%–5.6%, resulting in 3.4%–4.0% higher WUE, with the advantage reaching 4.8%–4.9% under I60. This advantage was associated with improved early sink establishment (higher spike numbers) and superior ET conversion efficiency, requiring 18.8% less ET per unit dry matter increment. Under I60 + 60, the relative advantage of SI narrowed, indicating diminishing marginal yield response to additional ET. A conceptual model was developed to synthesize these straw return-irrigation interactions. We identified SI-I60 + 60 (highest yield) and SI-I60 (highest WUE) as optimal coupling modes, and recommend SI-I60 as the optimal management practice for this region, offering a practical strategy for winter wheat production under water-limited conditions.