Moisture-dependent effects of PBS microplastics on cadmium mobility across soil aggregate fractions

The role of soil moisture in mediating the interactions between microplastics (MPs) and cadmium (Cd) in paddy soils, particularly their mutual effects on Cd availability is not yet fully understood and remains under debate. Soil samples were treated with two levels of polylactic acid (PBS) MPs (1% and 10% w/w) and incubated under 60% WHC (water holding capacity) and flooded conditions. The available Cd (ACd) content across different soil aggregate fractions, along with soil properties such as pH and iron oxide fractions, were analyzed. Correlation analysis and partial least squares path modeling (PLS-PM) were used to identify the key factors governing Cd availability. Results showed that the stability of soil aggregates in flooded soils was lower than that under 60% WHC. However, PBS MPs significantly enhanced aggregate stability and increased ACd contents, particularly in flooded soils. The highest ACd content was observed in the 1% PBS treatment under flooding, which showed a 23.3% increase compared to the control. PBS amendment notably elevated ACd levels within micro-aggregates. At the aggregate level, ACd was consistently positively correlated (p < 0.05) with PBS and moisture-influenced physicochemical properties. The PLS-PM model further demonstrated that PBS indirectly affected ACd by altering aggregate stability and iron oxide dynamics, with significant path coefficients of 0.215 and 0.721, respectively. Biodegradable MPs can enhance Cd mobility by disrupting soil structure and driving biogeochemical processes, thereby significantly increasing the associated ecological risks in flooded paddy fields.