Microplastic-induced changes in soil properties and microbial communities promote Cd and Pb release from soil aggregates

Microplastic (MP) and heavy metal co-contamination in soils have become an increasing global environmental concern due to its potential impacts on soil quality, ecosystem functioning, and contaminant cycling, yet the microbial mechanisms by which MPs regulate heavy metal mobilization within soil aggregates of different sizes remain unclear. Here, we investigated the effects of MP type, particle size and concentration on soil physicochemical properties, enzyme activities, microbial communities, and bioavailable Cd and Pb across different aggregate fractions. MP addition significantly decreased soil pH and increased Cd and Pb bioavailability, which increased by 0.6–17.0% and 0.8–14.0%, respectively, compared with the control, with microaggregates smaller than 0.25 mm showing the strongest response, although Cd decreased in the 0.25 mm fraction under PP treatment. MPs also increased the measured soil organic matter (SOM) pool, significantly altered the activities of enzymes involved in carbon, nitrogen, and phosphorus cycling, and MP exposure substantially reshaped bacterial and fungal community structures, suggesting substantial shifts in soil biochemical processes. Among the dominant microbial taxa, Actinobacteriota and Proteobacteria among bacteria, together with Ascomycota among fungi, were closely associated with Cd and Pb mobilization and exhibited pronounced aggregate size-dependent patterns. Partial least squares path modeling further showed that MPs indirectly regulated Cd and Pb bioavailability primarily by altering soil nutrient status, microbial biomass and microbial community composition. Overall, these findings demonstrate that MP-induced physicochemical and microbial alterations can promote Cd and Pb release from soil aggregates, particularly from microaggregates, providing new insights into the aggregate-scale mechanism underlying the potential ecological risks of MP–heavy metal co-contamination in soils.