Background We live in a plastic age (Thompson et al., 2009), with microplastic (typically defined as plastic particles 0.08 mm) generally enhance movement of particles, because sedimentation and sieving are not as pronounced, and they enhance the movement of water. This means that all players affecting the presence of macropores will indirectly influence the efficiency with which microplastic particles are moved in soil. The most important producers of biopores, macropores of typically tubular shape, are earthworms and roots. In addition, soil aggregation, a joint physiochemical/ biotic process, leaves macropores in between structural units, the aggregates. Experimental data for earthworms already exist (e.g., Rillig et al., 2017), even though in these studies active earthworms were present, and it is therefore not clear what percentage of microplastic particles moved through existing earthworm biopores rather than with the animals. There are no data on roots, however. Especially in agricultural systems, after harvest, this could be a massive transport pathway as roots decompose, leaving biopores. Root systems differ widely, for example in terms of depth and also in terms of fineness. One could expect that deeply rooted plants with coarser roots may be most effective at facilitating the movement of particles. Soil cracking and wet-dry cycles In agricultural soils with expanding mineral types, e.g., montmorillonite, cracks, and fissures can appear when soil dries. These cracks are open entryways for particles, that in this way could potentially move to substantial depths, very quickly arriving at deeper soil layers. Wet-dry cycles have been experimentally shown to directly mobilize colloid-sized particles in soils at a smaller scale (Majdalani et al., 2008), an effect the authors attributed to soil matrix weakening; similar patterns likely also hold for freeze-thaw cycles. Sequestration inside soil aggregates Microplastic particles will likely become embedded inside of soil aggregates, even though the extent to which this happens is unknown. Soil aggregation is a dynamic process, with aggregates being formed and disintegrating. During formation of macroaggregates in hierarchically structured soils, microplastic particles, and microaggregates (< 0.250 mm) will be included along with organic matter, microbes, and primary soil particles (Tisdall and Oades, 1982). During the persistence of macroaggregates, which can range range to weeks and months (Peng et al., 2017), the microplastic particles contained therein would be retained in the soil profile. Soil biota Other than as producers of biopores, soil biota can actively contribute to the movement of microplastic particles. Recently, microarthropods (collembola) have been shown to be able to move microplastic beads in a laboratory arena (Maaß et al., 2017). Such active, incidental, relatively small-scale transport could spread microplastic particles also horizontally, which may facilitate their subsequent entry into the soil. Similar effects can also be expected for mites, even though there is no experimental evidence yet. Perhaps fungal hyphae may also serve as preferential paths for movement of particles in the cm-range, as has been demonstrated for the transport of bacterial cells (Wick et al., 2007). The general literature on particle transport in soil by bioturbation (Gabet et al., 2003) also suggests that plant processes (e.g., root growth, uprooting) and various animals (earthworms, various larvae, vertebrates) can contribute to particle movement. Plowing and harvesting In agroecosystems, plowing is a widespread practice, and through this activity microplastic particles can be very effectively moved into the soil to the depth of the plow. Different tillage practices affect different soil layers and thus the depth to which microplastic can be incorporated. For instance, conventional tillage practices affect usually the first 20–30 cm, while in no-tillage soil disturbance, to place the seeds, affects only the very top soil layer, generally a few centimeters (Paustian et al., 1997). In addition, under conventional tillage different types of plowing may differ in the extent to which they facilitate microplastic incorporation along the layer affected by the machinery. Moaldboard plowing brings about an inversion of the respective soil layer, with the consequence that microplastic present at the soil surface will mostly be brought to a single layer at the plowing depth. By contrast, other tillage practices such as shallow hallowing or harrowing, have a mixing effect, likely resulting in the distribution of the microplastic particles throughout the tillage layer. Harvesting especially of plant portions below the soil surface (e.g., potatoes, carrots) can also serve to incorporate microplastic, albeit to a shallower depth, depending on the crop. Research needs and conclusions There are clear research needs that emerge from the discussion above: Dedicated column experiments in the laboratory, and eventually in the field, will be necessary to estimate rates of movement of microplastic particles, and to disentangle the relative roles of the various factors potentially influencing movement. This should include an assessment of risk for microplastics reaching groundwater. Such experiments and other studies should not only include beads or approximately spherical particles but also fibers and other plastic particles. There is very little we know about the behavior of microplastic fibers in soil, despite their likely prominence (e.g., Zubris and Richards, 2005; Hartline et al., 2016; Hernandez et al., 2017). Interactions with soil aggregates should be a focus, because microplastic particles could be incorporated into soil aggregates, thereby immobilizing these particles. However, this also likely protects microplastics from microbial breakdown, increasing overall residence time; and given the aggregate dynamics, particles would be continuously re-released. Additionally, it is unclear what effects microplastics have on the soil aggregation process itself, which could affect macropores and ultimately particle movement. Such future work is important: particles remaining at the surface could be moved around the landscape with potentially undesirable effects, but particles in the soil could have mostly unknown effects on soil biota and crop plants, possibly affecting food security. And, when microplastic particles move further through the soil profile, they would eventually also end up in groundwater. The contamination of subterranean waters with microplastic is of particular concern because they could have direct implications for human and animal health. Additionally, as a consequence of abrasion, chemical, or biodegradation occurring during transport, nanoplastic particles could be produced, posing fundamentally different hazards. Many aspects discussed here also pertain to soils in other terrestrial ecosystems; however, it is evident that the specific combination of machinery-driven soil preparation, crop cultivation, and harvest dynamics, and unique microplastic exposure pathways make agricultural soils particularly vulnerable and important to study. Author contributions MR: wrote the first draft of the paper; RI and AM: contributed ideas and text. Conflict of interest statement The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
