Despite great general benefits derived from plastic use, accumulation of plastic material in ecosystems, and especially microplastics, is becoming an increasing environmental concern. Microplastic has been extensively studied in aquatic environments, with very few studies focusing on soils.
Overview of the study on microplastics transport in soil:
Scientists tested the idea that microplastic particles (polyethylene beads) could be transported from the soil surface down the soil profile via earthworms.1 In this study scientists have begun testing microplastic movement into lower soil layers by analyzing how differing sized polyethylene beads moved in a 21-day period with and without earthworm facilitation.
Work by Matthias C. Rillig, Lisa Ziersch, and Stefan Hempel at Freie Universitet and Brandenburg Institute of Advanced Biodiversity Research in Berlin has been published in an article titled Microplastic Transport in Soil by Earthworms. This article investigates the effect of microplastics transport by earthworm’s movement into subsurface soil layers. 1
For this study scientists used clear, approximately spherical polyethylene (PE) microplastics (Cospheric, Santa Barbara, CA, USA), containing no additives or solvents (density 0.96 g cm−3). PE is among the major plastic types that are used worldwide. They assumed, based on a previous study, which also used PE, that earthworms might ingest and/or transport microparticles of different sizes to a different extent. For this reason, theyused four commercially available different particle size ranges: 710–850 μm, 1180–1400 μm, 1700–2000 μm and 2360–2800 μm. All particles were white in appearance, facilitating their retrieval from soil.1
The experiment was designed to confirm the assumption that earthworms would aid in particle movement. Results found earthworms to have a significant positive effect on transporting polyethylene particles from the soil surface. Particle size was also an important factor on the level of transportation into subsurface environments. With polyethylene spheres in the size range 710-850 micron (0.7-0.9mm) being significantly more likely to move into the lowest layer when earthworms were present.
Scientists were successfully able to observe and collect polyethylene beads in various regions of the soil as they were visible. The experiment demonstrated that the earthworms indeed did aid in the transport of the particles away from the soil surface to a variety of depth.
Even though the exact mechanism of the transport is unknown at this time, scientist highlighted the following implications:
Possible implications of effect of microplastics transport down the soil profile:
- Decomposition of organic material is generally much slower deeper in the soil, where microbial populations are much reduced; this would mean that microplastic, which is slow to turn over in the environment in the first place, may have even longer persistence once it reaches greater depths in the soil profile. This also highlights that as methods for quantifying microplastics in soil are being developed, not only should surface soils be tested, but also subsoils. It is presently unknown if microplastic makes significant contributions to soil organic carbon pools at various soil depths.
- Microplastic could potentially also reach groundwater, after passage through the soil profile, where it may then lead to undesirable effects akin to those already extensively documented in other aquatic environments.
- Microplastic that arrives in the soil may be subject to further disintegration, leading to the production of nano-sized material, which could have different functional properties and pose different environmental risks.1
Opportunities for further research on effect of microplastics:
With the increase in plastic usage in recent decades the issue of how this discarded plastic will affect marine environments has been studied extensively. However, effects of microplastics on soil environments have not been tested to the same extent.
With this experiment showing the ability of earthworms to transport microplastics into subsurface layers, more research needs to be done to determine the effect of microplastics transport on the soil environment and the worms themselves, including the multitude of other organisms that could also facilitate similar transportation, as well as the possibilities of microplastics reaching ground water where problems similar to those realized in marine systems could occur.
- Rillig MC, Ziersch L, Hempel S. Microplastic transport in soil by earthworms. Sci Rep. 2017;7(1):1362. Published 2017 May 2. doi:10.1038/s41598-017-01594-7