Divergent aging patterns of polylactic acid (PLA) and polyvinyl chloride (PVC) plastic sheets and their differential associations with soil carbon stability

Plastic pollution is a growing concern. Plastic particles undergo aging that changes their properties and ecological impacts, yet how different polymers age in natural settings and relate to soil carbon cycling remains unclear. We conducted a nine-month in situ experiment to compare the aging of biodegradable PLA (polylactic acid) and conventional PVC (polyvinyl chloride) sheets and to assess their associations with soil microbial communities, enzymes, and organic carbon fractions. PLA surfaces developed hydrolysis‑induced cracks and fragmentation, whereas PVC showed oxidative roughening. These aging patterns were associated with divergent functional potential of plastisphere microbial communities. PLA‑associated soils had higher β-glucosidase and peroxidase activities, together with a trend toward higher labile carbon (FPOC) indicators than PVC‑associated soils. Mineral‑associated organic carbon (MAOC) did not differ between treatments, but MAOC proportion was negatively correlated with bulk density only under PLA. Because our study lacked an unamended control, our conclusions are limited to PLA-PVC comparisons. Our findings show that the two polymers differ in aging pathways, microbial associations, and associations with labile carbon turnover that vary with soil physical properties, suggesting that the environmental implications of biodegradable plastics cannot be assumed from their label alone.