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Microplastic in Terrestrial Ecosystems and the Soil? Matthias C. Rillig* Freie Universität Berlin, Institut für Biologie, Plant Ecology, Altensteinstrasse 6, D-14195 Berlin, Germany given that the occurrence of larger plastic fragments in soils per se is nothing new, and has in fact been a trait used to describe urban soils or Technosols. Microplastic can occur in the environment either as primary or secondary microplastics.2 Particles can enter the environment directly as primary microplastic: microplastics are manufactured for a number of purposes, such as for use as industrial abrasives or for cosmetics products.2 In contrast, secondary microplastics are produced through the environmental degradation of larger-sized pieces. Secondary microplastics could result from abrasion of plastic debris at soil surfaces (where UV light could render the material brittle) or inside the soil profile. In agricultural fields where plastic mulching is practiced, an abundant source of plastic material would be available; in other cases, incidental plastic debris would be the starting material. Curiously, even washing machines can produce secondary microplastic fibers;3 via water treatment plants these could end up on agricultural fields. It is tempting to speculate that tumble driers could also be a possible source of microplastics. Very small particles or fibers could be spread further by becoming air-borne (for example from landfills, or other surface deposits) and then e live in a “plastic age” with more than 240 million tons enter terrestrial systems and the soil through atmospheric of plastic used annually, the majority of which for deposition. Geophagous soil fauna, most notably earthworms, disposable use.1 Due to limited recovery of discarded materials could contribute to secondary microplastic formation: in their and its durability, plastic debris is accumulating in the gizzard, they may grind up brittle plastic debris that they ingest environment. Recently, research on environmental impacts of into microplastic. Anecic earthworms, such that produce plastic has acquired a new dimension through the discovery and vertical burrows but feed near the soil surface, could even study of microplastic, particles often defined as smaller than 1 additionally promote incorporation of surface-deposited plastic mm, but that are often in the range of several micrometers.2 pieces into the soil. Other soil mesofauna (such as collembola These microplastics present a new set of issues, because of two or mites) may also contribute to this breakdown by incidentally main reasons: (i) they are small enough to be taken up by biota scraping or chewing off pieces of plastic. Also digging mammals, and thus can accumulate in the food chain; and (ii) they can such as gophers or moles, could conceivably contribute to sorb pollutants on their surfaces, thus enriching them on these abrasion and incorporation into the soil. particles. The occurrence of microplastic materials has been The actual direct, quantitative evidence of microplastic studied almost exclusively in marine environments and, related occurrence in soil is very thin. One study found synthetic to this, on shorelines. Yet the terrestrial landmasses are fibers in several soils in the U.S. to which organic waste material conspicuously empty on maps of global microplastic distribuhad been applied.5 Others have found spectra in their soil tion: they have simply not been studied.3 organic matter analyses that are consistent with the presence of Why have microplastics not been studied in soils and different types of plastic, but natural sources could not be terrestrial systems? First, there is a separation between marine completely ruled out. Many studies have just reported the and terrestrial ecological research such that ideas do not easily presence of plastic in soil, but have not quantified the amount, propagate from one domain to the other. Also important is the nor described the size of the particles. comparative ease with which microplastic filaments can be Given that microplastic likely is in our soils, can it have adverse effects? Obviously, soil is quite different from oceans, extracted and quantified from water. This is not so but soil also contains many features of an aquatic system: many straightforward for the complex organo−mineral soil matrix. biota are essentially aquatic, thriving in a thin film of water Also, there is a pattern of accumulation along shorelines, which covering soil surfaces. Thus, some of the same principles apply. has no parallel in terrestrial systems. Finally, aquatic environOf course the soil also has its filter feeders, active on the water ments harbor many filter-feeders, a mode of nutrition that will films on soil surfaces, like ciliates and rotifers; these similarly make organisms particularly susceptible to accumulating harmful particles from a large volume of the environment.4 Still, our lack of knowledge on microplastic in soil is surprising Published: May 31, 2012
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© 2012 American Chemical Society
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dx.doi.org/10.1021/es302011r | Environ. Sci. Technol. 2012, 46, 6453−6454
Environmental Science & Technology
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integrate over larger volumes of the environment, such as in bona fide aquatic systems. Microplastic could be ingested also by micro- and mesofauna, such as mites, collembola, or enchytraeids, and thus accumulate in the soil detrital food web. Microplastics could also sorb harmful contaminants from the soil solution and locally concentrate them in the soil. In addition, microplastics could alter physical properties of the soil. A key challenge will be to determine if the vast soil surfaces (mineral and organic) can sorb microplastic particles, and if they can be incorporated into the dynamic structure of soil aggregates, the crumbs in the soil, and thus rendered inaccessible to the biota. Plastic is quite resistant to degradation in soil. In fact, a standard test method for measuring biodegradation rates in soil employs polyethylene sheets as control. It will be quite crucial for understanding environmental impacts to study if microplastic, owing to its larger surface area, is more quickly degraded, or if it persists as long as in the oceans.2 In conclusion, occurrence of microplastics in soil is eminently plausible and should be systematically examined: this entails systematic quantification, preferably through development of spectroscopic methods (including infrared spectroscopy which has been frequently used in marine systems to identify particles). Once in the soil, these particles may persist, accumulate, and eventually reach levels that can affect the functioning and biodiversity of the soil and thus terrestrial ecosystems. With microplastics thus potentially being an issue “closer to home”, in our cities, for example, this topic may deserve increasing attention of policy makers and regulatory bodies.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; phone: +49 (0)30 83853165; fax; +49 (0)30 838-53886. Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Thompson, R. C.; Swan, S. H.; Moore, C. J.; vom Saal, F. S. Our plastic age. Philos. Trans. R. Soc., B 2009, 364, 1973−1976. (2) Cole, M.; Lindeque, P.; Halsband, C.; Galloway, T. S. Microplastics as contaminants in the marine environment: A review. Mar. Pollut. Bull. 2011, 62, 2588−2597. (3) Browne, M. A.; Crump, P.; Niven, S. J.; Teuten, E.; Tonkin, A.; Galloway, T.; Thompson, R. Accumulation of microplastic on shorelines worldwide: Sources and sinks. Environ. Sci. Technol. 2011, 45, 9175−9179. (4) Browne, M. A.; Dissanayake, A.; Galloway, T. S.; Lowe, D. M.; Thompson, R. C. Ingested microscopic plastic translocates to the circulatory system of the mussel, Mytilus edulis (L.). Environ. Sci. Technol. 2008, 42, 5026−5031. (5) Zubris, K. A. V.; Richards, B. K. Synthetic fibers as indicator of land application of sludge. Environ. Pollut. 2005, 138, 201−211.
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