Contrasting impacts of biodegradable and conventional microplastics on heavy metal bioavailability in soils: A multilevel meta-analysis.

The substitution of conventional non-biodegradable plastics (NBMPs) with biodegradable plastics (BMPs) has been widely advocated as a strategy to alleviate plastic pollution. However, whether this transition alters the geochemical behavior of legacy heavy metals (HMs) in soils remains unclear. Here, using a global dataset, we systematically compared the effects of NBMPs and BMPs on HM bioavailability, chemical speciation, and microbial ecological functions through multilevel meta-analysis and random forest modeling. NBMPs had negligible effects on overall HM bioavailability and speciation, indicating limited interference with soil HM dynamics. In contrast, BMPs significantly increased HM bioavailability, particularly for cationic metals such as Cd, and promoted their transformation toward more labile, acid-extractable fractions, whereas As mobility decreased. These responses were accompanied by coordinated changes in soil chemistry and microbial communities, including pH decline, dissolved organic carbon enrichment, enhanced enzyme activities, reduced microbial diversity, and a shift toward copiotrophic r-strategy taxa. Random forest analysis further identified pH, dissolved organic carbon, and the r/K strategy ratio as the most important predictors of HM bioavailability. Our results demonstrate that the environmental sustainability of BMP substitution should be evaluated not only by degradability, but also by its potential to disrupt soil geochemical balance and amplify ecological risks associated with legacy HMs.