Microscopic bits of plastic have been showing up throughout the environment. Most measurements of it have been made in water. But this pollution can taint the air and soil, too. A new study finds that microplastics in soil can stunt the growth of earthworms. And that’s worrying, its authors note, because earthworms tend to help make soil good for growing plants.
“Soils are the foundation of the food chain,” says Bas Boots. He’s one of the study’s authors. Healthy soil, he notes, “supports plant growth, including food we eat.”
Earthworm poop also helps keep soil healthy. As worms burrow, they eat organic matter such as dead plants. Their poop — called castings — contains essential nutrients such as nitrogen and phosphorous. These nutrients help plants grow. Boots works at Anglia Ruskin University in Cambridge, England. As an ecologist, he studies how people affect soils. He and his team focused on earthworms exposed to microplastics. Worms create little tunnels as they move through the ground. These tunnels allow air and water to get to plant roots. Without worm tunnels, soil can dry out and become too compacted for plants to grow well.
The new study found that worms living in soil with microplastics lost weight. So did ryegrass, a popular grass used for lawns. When ryegrass was grown along with worms and microplastics, both it and the worms became stunted.
Boots’ team shared its findings September 11 in Environmental Science & Technology.
A broad range of pollutant sources
Microplastics are five millimeters (0.2 inch) or smaller. They often are released by the breakdown of water bottles, shopping bags or other plastic products. Clothing made from nylon fleece also can shed plastic fibers when it’s washed. Those fibers will go down the drain and into the environment. Even car tires and breaks can release microplastics.
Scientists have known since the 1970s that microplastics pollute the ocean. Only in the last 15 years or so, have scientists learned how microplastics enter waterways, including lakes and rivers. (From there, those plastic bits flow to the oceans.)
How do they get there? Plastic fibers and dust can settle out of the air. Sewage can contain microplastics. Farmers sometimes spread that treated sewage on farm fields as fertilizer. Many farmers also cover parts of their fields with plastic mulch to suppress weeds and to keep soils from quickly drying out.
“People even throw plastic [trash] on the ground — including cigarette butts,” notes Boots. That plastic litter breaks down over time into smaller and smaller bits, he points out.
Earthworms make good gardeners
Studies have shown that when there are microplastics in water, fish and shellfish will eat them. Similarly, the new study finds that when there are microplastics in soil, worms will eat them. So the researchers designed experiments to learn how that might affect the worms.
First, Boots’ group added one of three types of microplastics to pots of dirt. Some pots got bits of polyethylene (Pah-lee-ETH-eh-leen). This material is used to make plastic bottles and disposable bags. Another set of pots got a biodegradable plastic made from polylactic acid. A third set had microplastic clothing fibers mixed into the dirt. Boots says the amount of microplastics his team added was “relatively high, but not unrealistic for soils.” A fourth set of pots had dirt with no plastic pollution.
Next, the researchers dug up rosy-tipped earthworms. They collected them from grassy areas in Cambridge. These worms are common in the topsoil of temperate regions (latitudes between the subtropics and the poles). Such regions include most of the United States, Canada and Europe, as well as Central Asia, southern South America and southern Australia.
The researchers weighed each worm at the start of the tests. That way they could compare its weight before and after exposure to the plastic. But first, they had to get the worms to empty their guts. Otherwise, the starting weights would include undigested organic matter. Then the worms’ end weights wouldn’t be a true measure of their growth.
Fortunately, worms are basically digesting and pooping machines, explains Boots. “The earthworm will just go about its business and will produce casts, or worm poo, as if it was still in the soil,” he says. “Without any soil to eat, this will empty their guts.” So the researchers just had to wait until the worms’ pooping stopped.
As they waited, the researchers also had to guard against the worms running away. “Earthworms can be surprisingly fast,” he says. So they corralled the worms during this stage in high-rimmed beakers.
Worms weigh in
Now the worms were ready to be tested. Boots’ team put two worms in each pot of dirt. Next, they planted the pots with ryegrass seeds. Then they let the worms munch away for 30 days. Afterward, the worms had a second weigh-in. Those living in plastic-free dirt gained an average of 5.1 percent over the month.
In contrast, all worms living in dirt with microplastics lost weight. On average, they dropped 3.1 percent of their starting weight. (Worms that ate polyethylene-loaded dirt lost the most — an average of 5.6 percent.)
Turf grass growing with plastic also was stunted when compared to the grass growing in clean dirt. The researchers could not say whether the stunted grass was due to the smaller worms in their pot or to toxic effects of the plastic. “That’s because we did not have a set of pots . . . without any earthworms,” says Boots. “So there is more work for us to do.”
The researchers also don’t know why the worms lost weight, but they have some ideas. The microplastic particles might have irritated their guts. Or chemicals in the plastic might have interfered with the worm’s ability to pick up nutrients from the dirt.
The new study is one of the first to look at the effect of microplastics in soil, notes Hayley McIlwraith. She studies microplastics at the University of Toronto in Canada. The use of one biodegradable type, here, is particularly interesting, she says. Many people don’t realize that biodegradable plastics can be harmful, she says.
Made from plants, biodegradable plastic is designed to break down more quickly than the traditional plastic made from oil. But biodegradable plastic will break down more quickly than regular plastic only if it is exposed to high temperatures for weeks at a time. This tends to happen only in special compost bins. In the open environment, it doesn’t break down any faster than regular plastic, she points out.
In that sense, she argues, “Biodegradable microplastics are not much better for the environment than regular microplastics.”
Boots hopes his study will encourage other scientists to do more research on the effect of microplastics in soil. “Earthworms are only a small part of the animal community that lives in the soil,” he points out. “If they are affected by plastics pollution, who knows what else may happen with others?”
acid A chemical term for materials that have a pH below 7.0 (on a 14-point scale). They are generally defined as “proton donors,” as their hydrogen atoms have a propensity for giving away their nuclei, which are lone protons. Other acids may not have protons and are instead defined as “electron-pair acceptors.” Acids often are capable of eating away at some minerals, such as carbonate, or preventing their formation in the first place.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
biodegradable The ability to break down, based on the activity of microbes, into simpler materials. This usually occurs in the presence of water, sunlight or other conditions that help nurture those organisms.
castings (in biology) The fecal wastes shed by worms.
chemical A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.
compost The end product in the breakdown, or decomposition, of leaves, plants, vegetables, manure and other once-living material. Compost is used to enrich garden soil, and earthworms sometimes aid this process.
earthworm A type of worm that lives in the soil. As it moves through soil, an earthworm creates burrows. These allow air and water to move more readily through the soil. The worms feed on decaying organic matter, which helps improve soil fertility.
ecologist Someone who works in a branch of biology that deals with the relations of organisms to one another and to their physical surroundings.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).
environmental science The study of ecosystems to help identify environmental problems and possible solutions. Environmental science can bring together many fields including physics, chemistry, biology and oceanography to understand how ecosystems function and how humans can coexist with them in harmony. People who work in this field are known as environmental scientists.
fertilizer Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.
fiber Something whose shape resembles a thread or filament. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fibers tend to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.
field An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.
gut An informal term for the gastrointestinal tract, especially the intestines.
latitude The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator; high latitudes are closer to the poles.
litter Material that lies around in the open, having been discarded or left to fall where it may. (in biology) Decaying leaves and other plant matter on the surface of a forest floor.
matter Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."
microplastic A small piece of plastic, 5 millimeters (0.2 inch) or smaller in size. Microplastics may have been produced at that small size, or their size may be the result of the breakdown of water bottles, plastic bags or other things that started out larger.
microscopic An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.
nitrogen A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth's atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or 14N and 15N).
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
nylon A silky material that is made from long, manufactured molecules called polymers. These are long chains of atoms linked together.
organic (in chemistry) An adjective that indicates something is carbon-containing; also a term that relates to the basic chemicals that make up living organisms. (in agriculture) Farm products grown without the use of non-natural and potentially toxic chemicals, such as pesticides.
particle A minute amount of something.
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
poles (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates. (in physics and electrical engineering) The ends of a magnet.
pollutant A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.
polyethylene A plastic made from chemicals that have been refined (produced from) crude oil and/or natural gas. The most common plastic in the world, it is flexible and tough. It also can resist radiation.
polylactic acid (abbreviated PLA) A plastic made by chemically linking long chains of lactic-acid molecules. Lactic acid is a substance present naturally in cow’s milk.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists
sewage Wastes — primarily urine and feces — that are mixed with water and flushed away from homes through a system of pipes for disposal in the environment (sometimes after being treated in a big water-treatment plant).
subtropics A geographic region that reaches to the beginning of the temperate climate zone (around 40° North and 40° South latitudes) from the edges of tropics, that band of hot climate spanning the outer belly of the planet. The tropics reach out to the Tropic of Cancer (around 23.5° north latitude) and Tropic of Capricorn (around 23.5° south latitude). The subtropics tend to be reliably warmer than temperate climates, but may experience brief periods of frost that would be unexpected in the true tropics.
taint To contaminate something with an unexpected, unnatural or illegal substance.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
temperate In geography, areas that are cooler than the tropics but warmer than polar regions.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
weed (in botany) A plant growing wild in, around — and sometimes smothering over — valued plants, such as crops or landscape species (including lawn grasses, flowers and shrubs). Often a plant becomes such a botanical bully when it enters a new environment with no natural predators or controlling conditions, such as hard frosts. (in biology, generally) Any organism may be referred to as a “weed” if it enters an environment and begins to overwhelm the local ecosystem.
Journal: B. Boots, C.W. Russell, and D.S. Green. Effects of microplastics in soil ecosystems: above and below ground. Environmental Science & Technology. Published online September 11, 2019. doi:10.1021/acs.est.9b03304.
Journal: S. Piehl et al. Identification and quantification of macro- and microplastics on an agricultural farmland. Scientific Reports. December 18, 2018. Vol. 8. doi: 10.1038/s41598-018-36172-y.