Biodegradable and non-biodegradable microplastics affect greenhouse gas emissions through chemical diversity and microbial biodiversity

While the environmental risks of soil microplastics (MPs) are well-established, their roles as exogenous carbon sources in driving greenhouse gas (GHG) emissions remain poorly understood. Particularly, the mechanisms by which biodegradable and non-biodegradable microplastics influence GHG emissions through microbial community shifts and soil organic carbon (SOC) chemical composition are unclear. To address this, this study investigated the impacts of two microplastics - biodegradable polylactic acid (PLA) and non-biodegradable polystyrene (PS), applied at 0.1 % or 1 % (w/w), on soil properties, carbon-related enzyme activities, GHG emissions, and microbial/chemical diversity. PLA addition significantly increased SOC and dissolved organic matter (DOM) content. Both microplastics stimulated lignin peroxidase and cellulase activities and shifted microbial composition: at the phylum level, Proteobacteria, Acidobacteriota, and Actinobacteria abundances changed; at the genus level, Vicinamibacterales, Vicinamibacteraceae, and Sphingomonas were altered. High-molecular-weight aromatic compounds increased under 1 % PLA and PS treatments. Microplastics elevated CO₂ and N₂O emissions but did not affect CH₄. Piecewise structural equation modeling revealed that GHG emissions correlated with chemical diversity (R²=0.45) and microbial diversity (R²=0.15). Our findings elucidate mechanistic links between microplastics-induced carbon transformation, microbial activity, and GHG emissions, highlighting distinct impacts of biodegradable versus conventional microplastics on soil-climate feedbacks.