Straw Return Enhances Photooxidative Disintegration of Mulch Film and Microplastics Formation in Farmlands

Plastic mulching is widely used in modern agriculture but represents a major source of microplastics in farmlands. Here, we show that straw return, a common practice to enhance soil fertility and crop productivity, can accelerate the disintegration of polyethylene mulch films. Using outdoor pot experiments spanning a full cropping cycle and controlled laboratory photoaging experiments, we demonstrate that amendment with maize straw or straw-derived biochar significantly enhances oxidation, physical damage, and fragmentation of black and white low-density polyethylene (LDPE) mulch films, substantially increasing microplastics formation. Mechanistic analyses reveal that compared with biochar amendment, straw return releases natural organic matter enriched in smaller, more aliphatic, and more hydrophilic components. This leads to markedly higher photochemical production of singlet oxygen, which attacks the amorphous domains of LDPE, promoting polymer chain scission and embrittlement. To contextualize potential agronomic implications, we use a canopy-scale model to link experimentally constrained microplastics levels to reduction in photosynthetic performance, showing maize yield losses exceeding 5% under representative conditions. Together, these results identify an overlooked interaction between residue management and agricultural plastics, demonstrating how straw-derived organic matter amplifies photooxidative degradation pathways that drive mulch fragmentation and underscore trade-offs that should be considered when designing sustainable straw return strategies.