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Contrasting impacts of biodegradable and conventional microplastics on heavy metal bioavailability in soils: A multilevel meta-analysis
Summary
"Biodegradable" plastics are often marketed as the eco-friendly alternative to regular plastic, but this study found they may actually make soil pollution worse: they caused toxic heavy metals like cadmium already lurking in soil to become more available for uptake by plants, by changing soil chemistry and microbial life. This matters because metals absorbed by crops can end up in our food, so switching to biodegradable plastics isn't automatically the safer choice, it may trade one environmental problem for another.
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.