Co-Occurrence and Interactions of Microplastics and Heavy Metals in Agricultural Soils: A Review

Agricultural soils are increasingly affected by the co-occurrence of heavy metals (HMs) and microplastics (MPs), but the conditions governing their combined behavior, metal mobility and bioavailability, remain insufficiently understood. This review synthesizes evidence on the co-occurrence, interaction mechanisms, environmental controls, and ecological iwct mplications of MP–HM co-contamination in agricultural soils. Available studies show that MPs can alter HM mobility and bioavailability through adsorption, surface complexation, ion exchange, and desorption. These processes are strongly influenced by soil pH, dissolved organic matter, polymer characteristics, aging status, clay and organic matter content, and microbial processes. Acidic conditions and dissolved organic matter can enhance metal release, whereas aged and oxidized MPs with oxygen-containing functional groups can strengthen metal retention under near-neutral to alkaline conditions. Co-contamination can also modify soil physicochemical properties, microbial communities, enzyme activities, and plant performance, including germination, growth, and oxidative stress responses. However, the direction and magnitude of these effects depend on MP properties, soil conditions, and other factors, indicating that MPs may function as either metal-retaining surfaces or mobile carriers rather than having a universal role. Biofilm-coated MPs may immobilize or mobilize metals depending on microbial community function, while direct field evidence remains limited because most mechanistic findings derive from laboratory systems. Potential human exposure may occur through soil-to-crop transfer, although risk depends on contaminant bioavailability, exposure pathway, level, and duration. A conditional framework is proposed to distinguish conditions favoring HM retention or redistribution and support soil-specific management and field research.