Earthworms multifacetedly drive size- and type-dependent microplastic transport in soils

Soil ecosystems are major sinks for microplastics (MPs), yet critical gaps remain in understanding how soil fauna mediate subsurface transport of MPs with different properties. This study reveals earthworms as multifunctional engineers driving MP redistribution through density-dependent bioturbation and material-selective interactions. Soil incubation experiments showed optimal vertical transport efficiency at 28.49 ± 5.70 earthworms/m², with depth-dependent burrowing creating biological channels. Small PET particles (45.8 ± 13.2 %) reached deeper soil layers (13.5-19.5 cm) more effectively than for larger fragments (27.80 ± 16.00 %). Earthworm pharyngeal filtration and ingestion preferentially mobilized submillimeter MPs (20-400 μm) and flexible fibers, achieving about 17-fold and 1.4-fold higher deep-layer accumulation than rigid particles. Mucus adhesion exhibited polymer-specific dynamics, with polypropylene MPs' low density overriding high hydrophobicity to enhance retention and film-shaped MPs showing greater retention than fibers or spheres. These findings highlight biologically moderated soil MP transport, necessitating risk frameworks that integrate faunal activity, polymer-specific interactions, and depth-dependent contamination thresholds for effective soil conservation strategies.