Microbial Community-Driven Cadmium Activation in High-Geochemical Background Soils by Small-Sized PBAT Microplastics

The activation of cadmium (Cd) in high geochemical background soils poses a significant threat to ecological safety. Microplastics (MPs) are known to alter soil physicochemical characteristics and microbial community structures, accelerating the transformation of stable Cd into bioavailable fractions. However, the specific effects of MP type, particle size, and concentration on Cd activation and translocation within plants in such soils remain unclear. In this study, a pot experiment was conducted using poly (butylene adipate-co-terephthalate) (PBAT) and polyethylene (PE) as model MPs to examine how particles with varying sizes (150 μm, 20 μm, and 5 μm) at three concentrations (0.1%, 0.5%, and 1% w/w) affect Cd behavior in naturally Cd-enriched soil. Compared with the control and PE treatments, 5 μm PBAT significantly increased soil pH and nutrient availability, resulting in an approximately 18% increase in bioavailable Cd. This enhanced mobility promoted Cd accumulation in lettuce roots by about 30%. Microbial community analyses revealed that PBAT markedly restructured microbial assemblages by increasing the relative abundance of Pseudomonadota and Basidiomycota while suppressing Actinobacteria, Bacillota, and Acidobacteriota, indicating a weakened microbial capacity for Cd immobilization. The restructuring of the microbial community was identified as the primary driver of Cd activation under 5 μm PBAT exposure, with specific taxa directly promoting Cd migration. Concurrently, elevated pH, nutrient release, and urease activity indirectly facilitated Cd mobilization through organic complexation and ionic competition. These findings underscore that small-sized biodegradable PBAT-MPs can intensify Cd activation and accumulation risks in high background agricultural soils, revealing an overlooked environmental concern.