Decrease in bioavailability of soil heavy metals caused by the presence of microplastics varies across aggregate levels.

Microplastics can alter the physicochemical and biogeochemical processes in soil, but whether these alterations have further the effects on the transformation of soil heavy metal speciation, and if so, whether these effects vary across soil aggregate levels remain unknown. Herein, long-term soil culture experiments and soil fractionation are combined to investigate the effects of microplastics on chemical speciation of Cu, Cr, and Ni with different particle-size soil aggregates. Results show that microplastics in soil decrease the exchangeable, carbonate-bound, and Fe-Mn oxide-bound fractions of metals but increase their organic-bound fractions via direct adsorption and indirect effects on the soil microenvironment conditions. The findings suggest that microplastics can promote the transformation of heavy metal speciation from bioavailable to organic bound. Such promotion exerts notable differences across soil aggregate levels. The transformation of soil heavy metal speciation is greater in larger aggregates than in smaller aggregates in the early incubation period with microplastics but shows the opposite trend in the later incubation period. Therefore, this process is more sensitive to long-term microplastic pollution in smaller aggregates than in larger aggregates, most likely owing to the lag in the influence of microplastics on metal speciation transformation in the smaller aggregates.