Sub-Soil Film Mulching Improves Rainfed Wheat Productivity and Water Use Efficiency and Alters Soil Hydrothermal-Saline Conditions in Coastal Saline–Alkaline Land

Biodegradable film is a potential alternative to polyethylene film, but its performance under sub-soil film mulching in saline–alkali soil remains unclear. A two-season winter wheat field experiment on the Bohai Coastal Plain compared a control without film mulching with polyethylene and biodegradable films on the soil surface or covered with approximately 1 cm of soil. Soil cover over film raised soil temperature before jointing and lowered it thereafter, while generally reducing electrical conductivity of the 1:5 soil–water extract (EC) and soluble Na+ in the 0–20 cm soil layer. Across materials and seasons, soil cover increased grain yield, maturity dry matter, and grain yield water use efficiency (WUEy) by 17.3%, 14.0%, and 22.4%, respectively, while estimated seasonal evapotranspiration was 4.8% lower. Piecewise structural equation modeling showed associations of soil cover with greater early-stage soil water storage and grain-filling dry matter. Greater grain-filling dry matter was associated with higher grain yield, whereas early-stage soil water storage did not significantly predict grain yield in the fitted model. Soil cover was associated with lower early-stage sodium adsorption ratio (SAR), whereas higher early-stage EC and SAR showed observational associations with lower grain-filling dry matter. The effect of soil cover on grain yield and WUEy did not differ significantly by film material, while the relative performance of the two film materials varied between growing seasons.