Effects of polylactic acid microplastics on dissolved organic matter across soil types: Insights into molecular composition

Increasing evidence has highlighted the effects of biodegradable microplastics (MPs) on soil organic matter (SOM), but the role of soil type and incubation time remains unclear. This study investigated the effects of polylactic acid microplastics (PLA-MPs) on the amount and molecular composition of dissolved organic matter (DOM) across three paddy soil types (Ferralsol, Alfisol, and Mollisol) and incubation times, revealing soil-specific patterns in DOM transformation: PLA-MPs reduced DOM content in Ferralsol and Alfisol by 29.3–68.2 mg/kg and 27.3–30.9 mg/kg, respectively, but initially increased it in Mollisol (30 d: 220.9 mg/kg; 60 d: 622.0 mg/kg). Molecular analyses revealed a decrease in DOM component diversity at both 30 and 180 d, potentially due to PLA-MPs stimulating microbial activity and accelerating native SOM decomposition. PLA-MPs promoted the formation of CHO (containing carbon (C), hydrogen (H), and oxygen (O)) compounds, whereas microbes selectively decomposed CHONS (containing C, H, O, nitrogen (N), and sulfur (S)) compounds to meet C and N demands, particularly in Ferralsol and Alfisol. This study enhances the understanding of biodegradable MPs’ impact on SOM, emphasizing the role of soil properties.