Biodegradable microplastics enhance heavy metal bioavailability in agricultural soils: Contrasting mechanisms, microbial responses, and mitigation strategies

Agricultural soils are increasingly recognized as major sinks for microplastics (MPs), which may coexist with heavy metals (HMs). The coexistence of MPs and HMs raises growing concern over soil health, reduced crop productivity, and potential risks to human health via the food chain. This critical review explores how biodegradable MPs (BMPs) and non-biodegradable MPs (NBMPs), in the presence of HMs, differentially affect soil physicochemical properties, HM adsorption–desorption, and metal bioavailability. BMPs can alter the soil microenvironment more strongly and negatively than NBMPs. BMPs increase HM bioavailability, disrupt enzyme activity and rhizosphere microbial communities, and often raise the relative abundance of taxa, with adverse effects on nutrient cycling and plant stress responses. In contrast, NBMPs are less reactive but tend to form more stable MP and HM complexes with lower desorption under acidic conditions and gradually affect plant root architecture, water uptake, and nutrient absorption by altering soil structure. These impacts may be mitigated using plant growth-promoting bacteria and metal-oxide engineered nanoparticles, which reduce contaminant stress and enhance antioxidant defenses. In addition, because BMP hydrolysis and oxidation can promote soil acidification and HM mobilization, alkaline biochar may help buffer acidity and reduce HM bioavailability. This study indicates that the effects of MPs and HMs co-exposure are strongly influenced by MPs degradability and concentration. Future studies should prioritize field-based investigations under environmentally relevant conditions and assess the long-term effectiveness and potential secondary ecological risks of amelioration strategies.