Earthworms transport microplastic fibres in soils

Microplastic (MP) contamination of agricultural soils is a growing concern. Synthetic textiles shed MP fibres throughout their lifecycle, which can end up in agricultural soils through the application of urban by-products as soil amendment. MP fibres can affect soil structure and have potentially adverse effects on soil organisms in high concentrations. Polyesters, such as polyethylene terephthalate (PET), are highly resistant to biodegradation and thus very persistent in soils. Understanding the fate and transport behaviour of MP fibres in soils is therefore essential for estimating long-term exposure levels and the potential effects of MP fibres on soil health. Fibres are considered less mobile in soil porewater compared to other particle shapes, but earthworms potentially displace or ingest and excrete fibres, even those within the millimetre size range. Hence, MP fibres may be prone to biologically driven transport despite their relatively large length.We therefore investigated whether earthworm burrowing causes vertical transport of relatively large MP fibres in soil. We measured the redistribution of MP fibres in laboratory-based process-studies introducing anecic earthworms (Lumbricus terrestris) to soil columns (30 cm depth) spiked with PET MP fibres of 1.3±0.7 mm length. The MP fibres were spiked to an upper surface layer of soil and doped with a metal tracer to facilitate detection with inductively-coupled plasma mass-spectrometry after acid extraction. MP fibre transport was monitored over a total of 4 weeks in different depth segments of the soil columns. We further analyzed the size distribution of MP fibres for the different depths using a visual microscope. At the end of the experiment, MP fibres were detectable in all depth segments, highlighting the transport potential of MP fibres by larger earthworms, such as L. terrestris. We also observed that the depth-dependent decline for MP fibres was stronger in comparison to previous studies with smaller particles proposedly because these are ingested more easily. Accordingly, we expect a relative enrichment of smaller MP fibres in the deeper soil layers. We conclude that biologically driven transport may overall be influenced, but less dependent, on particle length than other transport processes such as advective transport in soil pores. In the field, MP fibres will be exposed to bioturbation processes for much longer times than in this current study, likely resulting in a successive downward transport of even larger fibres depending on the local soil conditions and earthworm activity.