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Microplastics in Soils

Microplastics are a threat to more than just the oceans: Plant ecologist Matthias Rillig studies soil contamination.

Mar 17, 2017

A far-reaching problem: It takes a very long time for natural processes to degrade plastic, which is practically immune to chemical processes. The widespread use of plastic has thus become a serious problem for humanity at large.

A far-reaching problem: It takes a very long time for natural processes to degrade plastic, which is practically immune to chemical processes. The widespread use of plastic has thus become a serious problem for humanity at large.
Image Credit: aryfahmed / Fotolia

Plastic is practical, hygienic, and sturdy, and it can be used in every area of our lives. As a result, it is no wonder that about 320 million tons of plastics were produced worldwide in 2015 – and the figure is rising. But the fact that plastic is so stable, even practically inert – there are almost no substances that react chemically with plastic – is both an advantage and a problem. Unlike paper and other forms of organic waste, plastic waste biodegrades extremely slowly.

UV radiation and mechanical abrasion do break plastics down into smaller and smaller pieces over time. The result? Microplastics, tiny particles ranging in diameter from one to five millimeters, depending on the definition.

By now, everyone has heard of the vast patches of plastic waste floating in the oceans. Matthias Rillig, a plant ecologist at Freie Universität Berlin, has been studying this subject for some time now. He says, “There are already good studies of the global distribution of microplastics in the oceans and coastal regions, but practically nothing is known about the situation in terrestrial ecosystems.”

Rivers and lakes have already been shown to be polluted with microplastics, he explains. But Rillig believes the problem is much more widespread than that. “I firmly believe that microplastics have made their way into the soil by now,” Rillig says. “Everything that is produced industrially accumulates in the environment at some point,” he explains. He wrote his first article on this topic in 2012.

But what are the risks associated with microplastic particles? Plastics are not toxic per se, after all. “There’s practically nothing they react with themselves. But toxic substances can adhere readily to the surface of plastics,” Rillig explains. A group of researchers in Hamburg recently collected microplastic samples from a variety of sites, including the Weser, Elbe, and Jade river deltas and in Bodden areas along Germany’s Baltic Sea coast, and found that they were three to four times as contaminated with harmful substances as the surrounding sediment.

While the tiny particles are easily filtered out of water, it is much harder to separate them from soil samples. After all, soil is composed of practically nothing but particles. “It consists of clay, silt, and sand – along with countless organic compounds,” Rillig explains, adding that completely new methods would need to be developed before accurate analyses can be performed.

Microplastics are not only formed when large plastic products such as plastic bags break down. They are also deliberately produced by certain industries. In 2012, an estimated 4,360 tons of microplastics were produced in the EU. These microbeads are added to exfoliating skincare products, shower gels, and toothpaste, along with other cosmetic items, such as face powder, mascara, and lipstick.

That necessarily means the majority of them wind up in wastewater at some point. “That means one pathway into the soil is already certain: It is spread over the fields with sludge and then later worked deep into the soil by agricultural machinery,” Rillig explains.

Some countries have already taken action; it is illegal to produce or sell cosmetics that contain microplastics in the United States. The United Kingdom has announced a ban to take effect in late 2017, and the Netherlands and Canada have similar plans. In Germany, by contrast, producers are trusted to voluntarily refrain from using microplastics. That is something they can easily do, since ecofriendly substitutes are plentiful: Silica, healing clay, crushed apricot kernels, salt, and many other substances can be used.

According to calculations performed by environmental researchers, about the same amount of microplastics winds up on land as washes into the oceans. In some industries, plastics are even deliberately mixed into the soil. “Greenhouses use tiny expanded polystyrene beads to loosen the soil – and the plants just grow better,” as Rillig himself has observed. There are certain species of bacteria that can break down plastics, but Rillig says they are not efficient enough.

Many Ways for Plastic to Enter Our Food Chain

In Rillig’s working group, several bachelor’s theses are currently being written on the subject of microplastics. The students have found that earthworms and microarthropods spread the particles further into the soil. In fact, eating plastic isn’t particularly good for the earthworms; they no longer grow to their normal size, and some die. So far, however, no noteworthy effects of microplastics in the soil on microorganisms or plant growth have been observed.

Are microplastics in fields the next big environmental issue, or rather an academic question? Rillig isn’t sure yet. “I think we don’t yet know enough about this subject,” he says. What worries him is that microplastics could break down into smaller and smaller fragments over time, forming nanoplastics. “We know that plants absorb nanoparticles through their roots, and that they can reach as far as the leaves.” That would mean plastic was entering our food chain not only through fish and other seafood, but also through agricultural products – organic or not. Nanoplastics could also be inhaled as a fine dust, such as asbestos or pollen.

Rillig is continuing his research, driven by curiosity. “Maybe someday we will find out that microplastics accumulate for a couple thousand years, but then at some point they’re gone, and they haven’t had any noteworthy effects along the way,” he says. But research findings so far do not suggest any reason to stop being concerned.

Although the data are very sparse so far, plastic is essentially foreign to the natural environment. The less of it winds up in nature, the better. And that’s something each and every one of us can help to accomplish – by not using it.

This text originally appeared in German on February 18, 2017 in the Tagesspiegel newspaper supplement published by Freie Universität.