Biodegradable microplastics exert differential impacts from polyethylene on pesticide fate in soil-earthworm systems: insights into degradation selectivity and microbial mechanisms

This study investigated the effects of conventional polyethylene (PE) and biodegradable polylactic acid (PLA) microplastics on the degradation behavior and ecological impacts of three commonly used pesticides—dimethomorph, metolachlor, and imidacloprid—in a soil-earthworm system. Through controlled laboratory incubation experiments, a comprehensive approach combining soil physicochemical property analysis, earthworm biochemical assays, pesticide residue monitoring, and microbial community sequencing was employed to systematically assess the effects of microplastic type and concentration on pesticide degradation pathways, earthworm antioxidant responses, and bacterial community structure. The results demonstrated that a high concentration of PLA (1 % w/w) significantly enhanced the degradation rate of dimethomorph by 6.75 % and increased the accumulation of its Z -isomer in earthworms by 76.4 % compared to the control. Concurrently, PLA at 1 % elevated the accumulation of the metolachlor metabolite MOA by 33.29 % in soil and 82.5 % in earthworms, and induced oxidative stress in earthworms (e.g., a significant increase of approximately 50 % in MDA content) alongside intestinal microbiota dysbiosis. In contrast, PE exhibited relatively minor effects. Soils amended with PLA showed a notable enrichment of pesticide-degrading bacterial genera, such as Pseudarthrobacter and Rhodococcus , with relative abundances increased by 103 % and 65.4 %, respectively, and significant correlations were identified among environmental factors, earthworm physiological indices, and pesticide degradation rates. In conclusion, while biodegradable PLA microplastics can promote the transformation of certain pesticides, they may also increase ecological risks by disrupting bacterial communities and affecting earthworm health. This highlights the importance of conducting a comprehensive evaluation of their environmental safety. • Microplastics' impact on pesticide metabolites varies by type/concentration. • PLA induces earthworm oxidative stress and gut microbiota disruption. • Microplastics act as vectors for pesticide accumulation in earthworms. • PLA reduces soil microbial diversity but enriched degradative bacterial genera. • Polymer type and concentration critically drive microplastic-pesticide interactions.