Optimized Straw Strip Mulching Enhances Soil Water–Heat–Carbon Synergy and Stabilizes Winter Wheat Yield in Semi-Arid Regions

To address water-heat constraints and environmental risks associated with plastic film mulching in winter wheat production in the semi-arid region of Northwest China, a two-year field experiment (2021–2023) was conducted in Tongwei County, Gansu Province. A single-factor randomized block design was applied, with full plastic film mulching (PM) and bare land (CK) as controls, to evaluate the effects of 3-row (S3), 4-row (S4), and 5-row (S5) corn stalk strip mulching on soil hydrothermal conditions, active carbon fractions, and yield under rainfed conditions. Results showed that straw mulching significantly enhanced soil water retention, particularly in the 0–40 cm layer, where moisture content increased by 7.70–19.28% compared with CK (p < 0.05), with S3 performing best. Treatment S5 achieved the highest accumulated temperature and reduced the soil diurnal temperature range by 20.73–35.62% (p < 0.05). Active carbon fractions were also significantly improved, especially during the jointing–grain-filling stage (BBCH 31–87). In terms of yield, S5 exhibited the greatest increase, with a 15.88% higher two-year average grain yield than CK (p < 0.05), reaching over 90% of PM. Overall, S5 demonstrated optimal synergistic regulation of water, heat, and carbon, indicating strong potential as a sustainable alternative to plastic film mulching.