Microplastic in terrestrial ecosystems and the soil?

W live in a “plastic age” with more than 240 million tons of plastic used annually, the majority of which for disposable use. Due to limited recovery of discarded materials and its durability, plastic debris is accumulating in the environment. Recently, research on environmental impacts of plastic has acquired a new dimension through the discovery and study of microplastic, particles often defined as smaller than 1 mm, but that are often in the range of several micrometers. These microplastics present a new set of issues, because of two main reasons: (i) they are small enough to be taken up by biota and thus can accumulate in the food chain; and (ii) they can sorb pollutants on their surfaces, thus enriching them on these particles. The occurrence of microplastic materials has been studied almost exclusively in marine environments and, related to this, on shorelines. Yet the terrestrial landmasses are conspicuously empty on maps of global microplastic distribution: they have simply not been studied. Why have microplastics not been studied in soils and terrestrial systems? First, there is a separation between marine and terrestrial ecological research such that ideas do not easily propagate from one domain to the other. Also important is the comparative ease with which microplastic filaments can be extracted and quantified from water. This is not so straightforward for the complex organo−mineral soil matrix. Also, there is a pattern of accumulation along shorelines, which has no parallel in terrestrial systems. Finally, aquatic environments harbor many filter-feeders, a mode of nutrition that will make organisms particularly susceptible to accumulating harmful particles from a large volume of the environment. Still, our lack of knowledge on microplastic in soil is surprising 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. 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. 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; 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 enter terrestrial systems and the soil through atmospheric deposition. Geophagous soil fauna, most notably earthworms, could contribute to secondary microplastic formation: in their gizzard, they may grind up brittle plastic debris that they ingest into microplastic. Anecic earthworms, such that produce vertical burrows but feed near the soil surface, could even additionally promote incorporation of surface-deposited plastic pieces into the soil. Other soil mesofauna (such as collembola or mites) may also contribute to this breakdown by incidentally scraping or chewing off pieces of plastic. Also digging mammals, such as gophers or moles, could conceivably contribute to abrasion and incorporation into the soil. The actual direct, quantitative evidence of microplastic occurrence in soil is very thin. One study found synthetic fibers in several soils in the U.S. to which organic waste material had been applied. Others have found spectra in their soil organic matter analyses that are consistent with the presence of different types of plastic, but natural sources could not be completely ruled out. Many studies have just reported the presence of plastic in soil, but have not quantified the amount, nor described the size of the particles. Given that microplastic likely is in our soils, can it have adverse effects? Obviously, soil is quite different from oceans, but soil also contains many features of an aquatic system: many biota are essentially aquatic, thriving in a thin film of water covering soil surfaces. Thus, some of the same principles apply. Of course the soil also has its filter feeders, active on the water films on soil surfaces, like ciliates and rotifers; these similarly