Microplastic transport in soil by earthworms

Abstract

Despite great general benefits derived from plastic use, accumulation of plastic material in ecosystems, and especially microplastic, is becoming an increasing environmental concern. Microplastic has been extensively studied in aquatic environments, with very few studies focusing on soils. We here tested the idea that microplastic particles (polyethylene beads) could be transported from the soil surface down the soil profile via earthworms. We used Lumbricus terrestris L., an anecic earthworm species, in a factorial greenhouse experiment with four different microplastic sizes. Presence of earthworms greatly increased the presence of microplastic particles at depth (we examined 3 soil layers, each 3.5 cm deep), with smaller PE microbeads having been transported downward to a greater extent. Our study clearly shows that earthworms can be significant transport agents of microplastics in soils, incorporating this material into soil, likely via casts, burrows (affecting soil hydraulics), egestion and adherence to the earthworm exterior. This movement has potential consequences for exposure of other soil biota to microplastics, for the residence times of microplastic at greater depth, and for the possible eventual arrival of microplastics in the groundwater.

A catchment‐scale perspective of plastic pollution

“…Once in terrestrial ecosystems, plastics accumulated in surface soils and can be ingested by soil-dwelling organisms (Rillig, 2012;Rillig, Ingraffia, & Souza Machado, 2017). Empirical data indicate that plastics are incorporated into earthworm casts (Huerta Lwanga et al, 2017), and also that polyethylene microbeads (0.71-2.8 mm) reach down into the subsurface through earthworm burrows (Rillig, Ziersch, & Hempel, 2017). The concentration of plastic in soils varies; river floodplains across Switzerland revealed relatively low concentrations of microplastics (0-55.5 mg/kg, Scheurer & Bigalke, 2018), but more heavily contaminated industrial soils (300-67,500 mg/ kg) have been observed from samples collected in Australia (Fuller & Gautam, 2016).…”

Towards an ecology of soil microplastics

“…In addition to simply moving microplastics, biotic activities may also place them in pedological contexts they would rarely if ever otherwise occupy. Earthworms incorporate microplastics into their casts and burrow walls (Huerta Lwanga, Gertsen, et al, ; Huerta Lwanga et al, ; Huerta Lwanga, Mendoza Vega, et al, ; Prendergast‐Miller et al, ; Rillig, Ziersch, et al, ). These casts and burrows comprise a unique microhabitat that soil ecologists term the drilosphere (Andriuzzi, Bolger, & Schmidt, ; Sharma, Chandra, & Kanta, ).…”

“…Abiotic movement of microplastics through the soil matrix may be altered through biological activities. Earthworm burrowing increases soil porosity and water infiltration rates (Bottinelli et al, 2015;Capowiez et al, 2009;Lavelle et al, 1997), which may enable deeper water-assisted spread of microplastics into the soil profile (Rillig, Ziersch, & Hempel, 2017;Yu et al, 2017). Plant roots can also move soil organisms as they grow and create channels for additional movement (Demarta, Hibbard, Bohn, & Hiltpold, 2014;Dighton, Jones, Robinson, & Beckett, 1997) and thus may also facilitate passive microplastic dispersal deeper into the soil profile.…”

“…Though plastics obviously cannot actively hitchhike, they can still be passively dispersed on or inside soil organisms (Maaß et al, 2017;Rillig, Ziersch, et al, 2017;Zhu, Bi, et al, 2018). Although dispersal by springtails and mites has so far only been demonstrated in Petri dish arenas (Maaß et al, 2017;Zhu, Bi, et al, 2018), microplastics have been found in field-collected earthworm casts (Huerta Lwanga, Mendoza Vega, et al, 2017).…”

“…Within soil pores, springtail movement may also facilitate influxes of small microplastic particles (Kim & An, 2019 In addition to simply moving microplastics, biotic activities may also place them in pedological contexts they would rarely if ever otherwise occupy. Earthworms incorporate microplastics into their casts and burrow walls (Huerta Lwanga, Gertsen, et al, 2017;Huerta Lwanga et al, 2016;Huerta Lwanga, Mendoza Vega, et al, 2017;Prendergast-Miller et al, 2019;Rillig, Ziersch, et al, 2017).…”