Leaching of microplastics by preferential flow in earthworm (Lumbricus terrestris) burrows

Abstract

Environmental contextMicroplastics found in soil pose several potential environmental risks. This study shows that microplastics on the soil surface can be ingested by earthworms and transported to the lower soil layers. In this way, microplastics may enter the food chain and find their way into groundwater systems, especially in cases where the water table is shallow. AbstractIn the current study, we examine how the activities of earthworms (Lumbricus terrestris) affect microplastic (MP) distribution and concentration in soil, with a focus on low density polyethylene (LDPE). We also want to determine if MPs can be flushed out with water. We used a laboratory sandy soil column (polyvinyl chloride tube) experimental set-up and tested five different treatments: (1) treatment with just soil (control) to check if the saturated conductivity (Ksat) could be impacted by MP, (2) treatment with MP, (3) treatment with MP and litter, (4) treatment with earthworms and litter as a second control for treatment 5 and (5) treatment with MPs, earthworms and litter. Each treatment consisted of eight replicates. For the treatments with MP, the concentration of MP added at the start of the experiment was 7% by weight (3.97g, polyethylene, 50% 1mm–250µm, 30% 250µm–150µm and 20% <150µm) based on 52.78g of dry litter from Populus nigra. In the treatments using earthworms, two adult earthworms, with an initial average weight of (7.14±0.26) g, were placed in each column. Results showed that LDPE particles could be introduced into the soil by the earthworms. MP particles were detected in each soil sample and within different soil layers for the earthworm treatments. Earthworms showed a tendency to transport the smaller MP particles and that the amount of MPs in size class <250μm increased in soil samples with increasing soil depth in comparison to the other size classes. After leaching, MPs were only detected in the leachate from the treatments with the earthworms, and the MP had similar size distributions as the soil samples in the 40–50cm layer of the treatment with MP, earthworms and litter. The results of this study clearly show that biogenic activities can mobilise MP transport from the surface into the soil and even be leached into drainage. It is highly likely that biogenic activities constitute a potential pathway for MPs to be transported into soil and groundwater.

Towards an ecology of soil microplastics

“…Lehmann, Fitschen, and Rillig () demonstrated that microplastic effects on abiotic soil properties were partially mediated by soil microbial communities, and their effects on biotic communities may be likewise dependent on those communities themselves. For example, microplastics could have radically different effects in a European forest soil with a diverse assemblage of native earthworms than in a similar North American soil in a remote area where introduced earthworms have yet to reach, given what earthworms and their gut microbiota have been shown to do to microplastics, dispersing them through soil and potentially reducing their size through partial biodegradation (Huerta Lwanga, Gertsen, et al, ; Huerta Lwanga et al, ; Yu et al, ). We find the following questions raised by the current state of knowledge: What organisms can facilitate microplastic formation from larger plastic debris, to what extent and under what circumstances? Once microplastics are present in a soil (regardless of source), what organisms contribute to their dispersal throughout the soil profile, either vertically (deeper into the ground) or horizontally (away from a point source such as a piece of large plastic debris)? To what extent are the shapes and sizes of microplastic particles influenced by microbial or animal biodegradation or physical damage due to passage through animal guts? Can organisms besides earthworms create circumstances of unique microplastic positioning in soil that could not arise through abiotic processes, and what are the consequences of this for other organisms? To what extent can one organism's effect on microplastic size, shape or distribution in soil alter their effect on other organisms? …”

“…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.…”

“…Lehmann, Fitschen, and Rillig (2019) demonstrated that microplastic effects on abiotic soil properties were partially mediated by soil microbial communities, and their effects on biotic communities may be likewise dependent on those communities themselves. For example, microplastics could have radically different effects in a European forest soil with a diverse assemblage of native earthworms than in a similar North American soil in a remote area where introduced earthworms have yet to reach, given what earthworms and their gut microbiota have been shown to do to microplastics, dispersing them through soil and potentially reducing their size through partial biodegradation (Huerta Lwanga, Gertsen, et al, 2017;Huerta Lwanga et al, 2018;Yu et al, 2017). We find the following questions raised by the current state of knowledge: Answering these questions will require inventive experimental designs, often involving multiple soil organisms simultaneously, as well as further refinement of methods to extract and identify microplastics in soil and invertebrate samples.…”

Viscosity Controls Rapid Infiltration and Drainage, Not the Macropores

“…Their much weaker relative concentrations vs. drag force showed pronounced, yet undisclosed hysteresis. Recently, [63] called for improving our understanding of the transport of microplastic particles in soils, while [64] found these particles in worm burrows and assumed preferential flow along macropores as the major transport process. Thus, it might be worthwhile to further develop Newton's shear flow application to the particle transport under particular attention of the films thickness F and their specific contact areas L.…”

Foreword to the research front on ‘Microplastics in Soils'

“…The role of microplastics as vectors for organic micropollutants has been widely studied in aquatic systems, but much less so in soils. Yu et al (2019) investigate whether earthworms (Eisenia fetida) could be carriers of pesticides in these media. Interestingly, they could not demonstrate that microplastics can be carriers of pesticides to earthworms, but found that these organisms avoided pesticide-contaminated microplastics.…”