The deep-burrowing earthworm Lumbricus terrestris ingests and transports microplastic fibres of a wide length range in soils

Microplastic (MP) exposure of the terrestrial environment is increasingly reported, but exposure levels may change due to transport processes. MPs occur in different shapes. Particularly MP fibres can affect soil structure and soil organisms. Earthworms are important contributors to particle movement in soils, yet their influence on the redistribution of MP fibres remains poorly understood. This study investigated if the deep-burrowing earthworm Lumbricus terrestris enhances vertical MP fibre transport and whether fibre length affects ingestion and transport distance. We measured the mass-based redistribution of MP fibres by L. terrestris using repacked soil columns spiked with metal-doped polyethylene terephthalate (PET) fibres (median length 0.750 mm). Additionally, number concentrations and MP lengths were determined by optical microscopy. The transported MP mass-fraction increased from two to four weeks (5.7 to 9.0 % of MP mass), with most transport occurring during the first two weeks. L. terrestris preferentially transported smaller MP fibres, indicated by a depth-dependent decrease in MP fibre lengths, likely due to easier ingestion. However, absolute differences in MP fibre lengths across depth (<0.170 mm) and effect magnitudes were small (Cohen’s d < 0.2). Another experiment with homogeneously spiked soil confirmed instead that this earthworm species can ingest long MP fibres (up to 4.8 mm in casts; median 0.700 mm) that are otherwise often considered immobile in soils. The observed transport underscores that bioturbation is a relevant transport mechanism leading to a vertical redistribution of MPs entering soils. A broader range of bioturbating organisms in more complex systems need to be considered for establishing realistic transport rates.