Biodegradable soil film mulching enhances maize yield via coordinated regulation of soil physicochemical environments, microbial enzyme activities and nitrogen transformation

Soil film mulching refers to spraying carboxymethyl cellulose ethers onto the soil surface to form a dense film upon drying. However, the mechanisms governing crop productivity under this practice remain poorly understood. To address this, we conducted a three-year field experiment in dryland agriculture using a split-plot design with three materials (carboxymethyl cellulose ammonium, CMC-NH₄; carboxymethyl cellulose sodium, CMC-Na; and water) and two application methods: spraying (inducing film formation) and row application (without film formation). As the two water-only controls showed no film formation and negligible differences, they were merged into a single control (CK), yielding five treatments: CK, spraying of CMC-NH₄ (AS), row application of CMC-NH₄ (AR), spraying of CMC-Na (BS), and row application of CMC-Na (BR). Compared with CK, spraying treatments (AS and BS) significantly increased soil temperature by 2.26% and 2.56%, respectively. AS also increased soil volumetric water content by 8.78% relative to CK, whereas row application (AR and BR) decreased it by 21.5% and 16.2%, respectively. AS, BS, and BR increased maize yield by 30.9%, 20.9%, and 20.5%, respectively. Spraying treatments significantly increased the activities of key soil enzymes (β-1,4-glucosidase, urease, neutral protease, and nitrite reductase), while row application decreased those of dehydrogenase, cellulase, and nitrate reductase. All treatments significantly increased net N mineralization rate, ranging from 39.7% (BR) to 90.7% (AS) relative to CK, while net nitrification rate increased by 74.7% (AS) and 62.6% (BS), respectively. Structural equation modeling revealed that mulching enhanced soil physical and chemical environments, which regulated microbial enzyme activity and N-cycling gene profiles to drive N transformation rates, ultimately exerting a strong positive effect on maize yield. These findings provide a theoretical foundation and practical pathway for sustainable soil management in dryland agriculture.