Contrasting roles of soil aggregates in decoupling solid-phase available As from water-phase risk under microplastic disturbance.

Current assessments of arsenic (As) risk in agroecosystems rely on total mass or static solid-phase speciation, yet these metrics cannot capture the actual transfer of As into porewater and its subsequent export through leaching. Under persistent microplastic disturbance, how aggregate-scale heterogeneous microenvironments govern phase-to-phase As transport across the soil-water interface remains unresolved. Here, column leaching experiments were used to trace As migration among solid-phase available pools, porewater, and leachate in bulk soil, large macroaggregates (LMA), and small microaggregates (SMA) exposed to polyvinyl chloride microplastics. The results show that soil aggregates play contrasting roles in governing aqueous As transport, with LMA driving early release and SMA sustaining late-stage exposure. Under As-amended conditions, microplastics further decouple apparent stabilization of solid-phase available As from aqueous mobilization and export, as reduced solid-phase As availability coincides with elevated porewater and leachate As. This contrast becomes more pronounced during leaching, where intra-aggregate mobility is translated into actual export. Overall, under As-amended conditions, apparent decreases in solid-phase available As under microplastic disturbance do not necessarily indicate lower environmental risk, but may instead coincide with enhanced water-phase export. These findings move As assessment beyond static solid-phase proxies toward a continuous solid-phase available As-porewater-leachate framework.