Microplastic Incorporation into Soil in Agroecosystems

Background: We live in a plastic age (Thompson et al., 2009), with
\nmicroplastic (typically defined as plastic particles < 5 mm) becoming an
\nincreasingly appreciated aspect of environmental pollution. Research has been
\noverwhelmingly focused on aquatic systems, especially the oceans, but there is
\na current shift to more strongly consider terrestrial ecosystems (Rillig,
\n2012; Horton et al., 2017). In particular agroecosystems are coming into focus
\nas a major entry point for microplastics in continental systems (Nizzetto et
\nal., 2016b), where contamination might occur via different sources as sludge
\namendment or plastic mulching (Steinmetz et al., 2016). Given the central role
\nof agroecosystems, including their soil biodiversity (Rillig et al., 2016), in
\nfood production, such numbers are potential cause for concern. Field data on
\nmeasured microplastic presence in agricultural soils are still not widely
\navailable, but nevertheless this material is certain to arrive at the soil
\nsurface. The fate of material deposited at the soil surface is not clear:
\nparticles may be removed by wind or water erosion, becoming airborne, or may
\nbe lost by surface runoff (Nizzetto et al., 2016a). Nevertheless, a
\nsubstantial part of the microplastic (or nanoplastic following further
\ndisintegration) is expected to enter the soil. The degree of hazard
\nrepresented by microplastic to various soil biota is not clear. Direct
\nevidence comes from experimental work on earthworms, on which microbeads had
\nnegative effects (Huerta Lwanga et al., 2016; also reviewed in Horton et al.,
\n2017). Data on impacts on other soil biota groups are not available. However,
\nKiyama et al. (2012) have shown that polystyrene beads can be taken up by the
\nnematode Caenorhabditis elegans; this means the material could also accumulate
\nin the soil food web (Rillig, 2012). Movement into soil is an important aspect
\nof assessing risk: will soil biota be exposed to microplastics? Here, we
\nsketch what is known about movement of such particles in soil, which players
\nand factors could influence this, and we chart avenues for research aimed at
\nthe movement and distribution of microplastic in agricultural soils.