Carboxymethyl Cellulose Ammonium (CMC-NH4)-Based Soil Film Mulching Modulates Crop Yield by Altering Soil Hydrothermal Dynamics and Microbial Community

Soil film mulching can improve crop productivity, yet the relative importance of hydrothermal, nutrient, and microbial pathways remains unclear. In this spring wheat–summer maize rotation field experiment conducted in the Yellow River Irrigation District, we evaluated the effects of a soil film formed from carboxymethyl cellulose ammonium (CMC-NH4) solution on crop yield, soil properties, and microbial communities. The results showed that soil film mulching enhanced soil temperature and moisture retention, with greater CMC-NH4 application rates intensifying these hydrothermal improvements. Soil moisture played a more important role than soil temperature in promoting yield in both crop seasons. Bacterial community structure shifted markedly, with α diversity initially increasing before declining and β diversity showing a decreasing trend over time. PLS-PM further revealed contrasting pathways between the two crops, with hydrothermal conditions dominating spring wheat yield and hydrothermal and chemical pathways contributing comparably to summer maize yield. These findings indicate that soil film mulching enhances crop yield, with the dominant pathway differing between spring wheat and summer maize. This suggests that film application should be managed according to crop season and local water and nutrient conditions, offering a practical option for sustainable crop production in irrigated regions of the Yellow River Irrigation District.