Effect of biodegradable microplastics and Cd co-pollution on Cd bioavailability and plastisphere in soil-plant system

Biodegradable plastics (BPs) are regarded as ecomaterials and are emerging as a substitute for traditional non-degradable plastics. However, the information on the interaction between biodegradable microplastics (BMPs) and cadmium (Cd) in agricultural soil is still limited. Here, lettuce plants were cultured in BMPs (polylactic acid (PLA) MPs and poly(butylene-adipate-co-terephthalate) (PBAT) MPs) and Cd co-polluted soil for 35 days. The results show that diffusive gradient in thin films technique (DGT) but not diethylenetriaminepentaacetic acid (DTPA) extraction method greatly improved the prediction reliability of Cd bioavailability in non-rhizosphere soil treated with BMPs (R² = 0.902). BMPs increased the Cd bioavailability in non-rhizosphere soil indirectly by decreasing soil pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC), rather than by directly adsorbing Cd on their surface. PLA MPs incubated in rhizosphere soil showed more considerable degradation with extremely obvious cavities and the fracture of ester functional groups on their surface than PBAT MPs. BMPs could provide ecological niches to colonize and induce microorganisms associated with BMPs’ degradation to occupy a more dominant position. In addition, Cd only affected the composition and function of microbial communities in soil but not on BMPs. However, co-exposure to BMPs and Cd significantly reduced the degrees of co-occurrence network of fungal communities on PLA MPs and PBAT MPs by 37.7% and 26.7%, respectively, compared to single exposure to BMPs.