Earthworm Casting Drives Soil Microplastic Upward Transport and the Formation of Biogenic Polymer Aggregates

Earthworms act as key ecosystem engineers influencing the distribution of soil microplastics (MPs), however, the residence, transport, and fate of these particles within the drilosphere, particularly within biogenic cast aggregates, remain poorly understood. Here, we combined a field survey of 43 paired soil-cast samples across three agricultural land-use scenarios with complementary laboratory soil column experiments to elucidate earthworm-driven MP dynamics. The field survey revealed ubiquitous in situ MP occurrence and upward fluxes from bulk soils to casts, with transport efficiency modulated by soil clay and organic carbon contents. Laboratory simulations using epigeic and anecic species validated that different ecotypes actively ingest MPs from source soils and deposit them in surface casts. Crucially, both field and laboratory data demonstrated a significant reduction in particle size in casts compared to soil (6.48% and 19.8%, respectively), supporting the potential earthworm effects on MP mechanical attrition. Polymer compositions in casts mirrored those in soils, exhibiting a nonselective and passive ingestion pathway. Beyond physical transport, the formation of field biogenic polymer aggregates facilitated chemical aging of MPs, as evidenced by elevated oxidation indices. This process was likely accelerated by the enrichment of plastic-degrading microbial taxa (e.g., Flavobacterium) within casts, which exhibited up to a 35.6-fold increase in relative abundance. Collectively, these findings highlight the dual role of soil-engineering invertebrates in driving the vertical redistribution and physicochemical degradation of MPs in agricultural systems.