Microplastic–cadmium co-contamination in agricultural soils: A critical review of interactions, transfer, and plant exposure

Microplastics (MPs) and cadmium (Cd) are persistent contaminants in croplands, introduced through plastic residues, organic amendments, irrigation, fertilizer impurities, and atmospheric deposition. MPs are increasingly recognized not only as passive carriers of Cd, but also as active regulators of Cd behavior across the soil–rhizosphere–plant continuum. By altering surface reactions, aggregate structure, dissolved organic matter, microbial activity, and rhizosphere conditions, MPs can reshape Cd speciation, mobility, plant uptake and translocation. However, current evidence remains highly inconsistent. Across seemingly similar studies, MPs have been reported to enhance, suppress, or exert little effect on Cd bioavailability, plant accumulation, and toxicity. This inconsistency reflects not only environmental complexity, but also the lack of a unified framework linking MP heterogeneity with Cd regulation across soil processes, rhizosphere interactions, and plant internal transport. Here, we review MP–Cd co-contamination as a coupled perturbation rather than the simple sum of two pollutants. We synthesize how MPs regulate Cd behavior in soils, how these changes are translated into biological exposure at the rhizosphere interface, and how plant uptake, redistribution, and toxicity are subsequently modified. We further evaluate whether MP-mediated Cd responses can be interpreted within conventional soil predictors of Cd phytoavailability and soil-to-plant transfer or whether they imply an additional regulatory dimension. Finally, we identify key research priorities, including standardized characterization of MP–Cd systems, direct measurement of rhizosphere pH, dissolved organic carbon, microbial activity, and Cd accessibility, improved resolution of spatiotemporal dynamics, and quantitative partitioning of soil-, rhizosphere-, and plant-level controls.