Microplastics in Farm Soil and Our Food
The amount of microplastics in farm soil—and our food supply is growing alarming. More microplastics are contaminating agricultural lands than oceans, impacting plant development and ending up in produce and people. Cadmium and other toxic trace elements increase when there are particulate plastics in soil.
Plant physiologist and professor of agronomy at Kansas State University, Mary Beth Kirkham conducted her own research in which she cultivated wheat plants exposed to microplastics, cadmium, and both microplastics & cadmium. Then she compared these plants to those grown without either additive. She chose cadmium because it’s poisonous, carcinogenic. It is also ubiquitous in the environment due to human activity. Cadmium is shed from batteries and car tires. It is also naturally found in the phosphate rock used to make agricultural fertilizers. It is everywhere.
Kirkham’s expertise is in water and plant relations and heavy metal uptake. At the end of the experiment she sent her wheat plants off for analysis. In validation to previous reports, the plants grown with microplastics showed heavier cadmium contamination. The plastics were acting as the vector for uptake of the cadmium. Kirkham’s experiment became a chapter in her book, “Particulate Plastics in Terrestrial and Aquatic Environments.” Thus far, people just haven’t felt that microplastic uptake by plants is an issue. It just hasn’t been in the public eye.
Microplastics-loosely defined as plastic pieces smaller than 5 millimeters across (roughly the size of a small grain of rice)-have made their mark on both the global ecosystem and the popular consciousness. Famously killing seabirds and raining down on wilderness areas. And while the impact of ocean microplastics has been the subject of more media and scientific attention, researchers say that most microplastics are actually accumulating on land, including agricultural areas. The Environmental Science & Technology (EST) journal of the American Chemical Society (ACS) estimates that between 107,000-730,000 tons of microplastics are dumped onto farm lands in the U.S. and Europe, compared to the 93,000-236,000 tons that enter the oceans each year.
Microplastics arrive on farms through processed sewage sludge used for fertilizer, plastic mulches. They are even added to soil as slow-release fertilizers and protective seed coatings. In just the last few years, an increase in research has uncovered alarming potential impacts of this contamination on all aspects of agricultural systems from soil quality to human health.
Sewage Sludge, Mulch, and Slow-Release Fertilizers
Research scientist at the Norwegian Institute for Water Research (NIVA), Luca Nizzetto, began studying microplastics in agricultural soils after he noticed that most research on microplastics focused on oceans; yet most of the marine sources were land-based. It seems that no one is looking at what is happening close to the source. When his team began evaluating potential land sinks for microplastics they immediately identified agriculture as one of the hot areas.
Microplastics enter agricultural lands via sewage sludge, the solids that are filtered out of wastewater. These solids are commonly used to fertilize agricultural fields. Microplastics get into the wastewater originally through laundry, personal care products, and urban runoff. Most of the microplastics get retained in the sludge as the water is cleaned in treatment facilities.
In “Are Agricultural Soils Dumps for Microplastics of Urban Origin?” Nizzetto and his team estimate that between 125 and 850 tons per million microplastics are annually dumped on European agricultural lands via sewage sludge. The same paper reports that roughly 50 percent of sewage sludge is processed for agricultural application in both Europe and the United States.
In a Springer Nature Switzerland, AG article, “Synthetic Fibers as Indicators of Municipal Sewage Sludge, Sludge Products, and Sewage Treatment Plant Effluents,” microplastics were reported in U.S. sewage sludge as early as 1998. In 2020 researchers estimated roughly 21,249 metric tons of microplastics are released to U.S. agricultural lands from sewage sludge annually. Because of their recalcitrance (refusal to move, budge) in soils, U.S. researchers, in “Synthetic fibers as an indicator of land application of sludge,” investigated the possibility of using the contemporary microplastics profile of soils as an indicator of past sewage sludge application.
Microplastics can also enter agricultural soils through the degradation of plastic materials used by farmers. Kirkham said that in the 1950s, plastic covering replaced glass in greenhouses. Plastic mulches have also popularized, becoming commonplace around the world. These mulches (sheets of plastic laid out on the ground to suppress weeds, warm the soil, and retain moisture) are challenging to recycle and costly to dispose of.
According to Kirkham, farmers may end up piling them up on their land or burning them to avoid disposal costs. In some areas, the mulches are simply left to break down into the soil.
Intentionally manufactured microplastics are another source of microplastic emissions to agricultural soils, according to Nizzetto and other researchers. These can include plastic encapsulated slow-release fertilizers and plastic coatings intended to protect seeds from microorganisms.
A 2017 report compiled for the European Commission estimates that every year, up to 8,000 metric tons of plastic from slow-release fertilizers are spread to European agricultural soil. Although a percentage of this may not be microplastics. A 2019 European Chemicals Agency (ECHA) report listed emission amounts as 10,000 metric tons for slow-release fertilizers, and 500 metric tons for treated seeds every year. Figures for the U.S. were not available.
Microplastics Alter the Physical and Biological Properties of Soils
Sixteen days into Kirkham’s microplastics and cadmium experiment, her plastic-treated wheat plants began to yellow and wilt. Water ended up pooling on the top soil in the plastic-treated plants; but to keep her experiment consistent, she had to give all the plants the same amount of water.
The particulate plastic appeared to clog the soil pores, prevent aeration of the soil, and cause the roots to die. The plants without microplastics, even the cadmium-contaminated ones, were in much better shape. The plastics were controlling the growth more than the cadmium.
In “Differentially charged nanoplastics demonstrate distinct accumulation in ‘Arabidopsis thaliana’‘,” reported similar results. Scientists found that exposure to plastics resulted in reduced weight, height, chlorophyll content, and root growth of Arabidopsis thaliana, a cruciferous relative of cabbage and broccoli. In this study, researchers used nanoplastics, which are plastic pieces that are less than 100 nanometers in size. For scale, the novel coronavirus measures 60 to 140 nanometers.
The full impact of microplastics contamination in agricultural soils, particularly as concentrations increase with time, is unknown. However, another study, “Impacts of Microplastics on the Soil Biophysical Environment,” showed that microplastics possess physical and chemical characteristics with the potential to alter soil bulk density, microbial communities, water holding capacity, and other properties influencing plant development.
One study, “Microplastics negatively affect soil fauna but stimulate microbial activity: insights from a field-based microplastic addition experiment,” published in The Royal Society, found that adding low-density polyethylene (LDPE) fragments to soil significantly affected the composition and abundance of micro fauna. It also found that the effect of those fragments could cascade through the soil food web, with potential consequences on soil carbon and nutrient cycling.
Earthworm Impacts
Esperanza Huerta Lwanga is a soil scientist affiliated with both Wageningen University & Research in the Netherlands, and El Colegio de la Frontera Sur in Mexico. She has investigated the effects of microplastics on earthworms, creatures widely considered a boon to farming due to their ability to aid decomposition, add organic nutrients to the soil through their waste castings, and increase the aeration of soil.
When doing research on soil invertebrates’ distribution at different home gardens in Tabasco, Mexico, Huerta Lwanga found microplastics. And in those soils with microplastics, there were not earthworms. This observation motivated her to study earthworms directly. In her subsequent experiments, she found that worms attempted to avoid microplastics. But when the soil concentration reached 7 percent, they began to ingest them along with the soil. Concentrating the plastics in their castings, and transporting them through different layers of soil. In the Australian Academy of Science Commonwealth Scientific and Industrial Research Organisation’s (CSIRO Publishing) “Leaching of microplastics by preferential flow in earthworm (Lumbricus terrestris) burrows,” Huerta Lwanga’s team cautioned that rainwater flows through earthworm burrows into groundwater, creating a clear conduit for microplastics to enter groundwater systems.
Huerta Lwanga also said that microplastics caused an 8-25 percent mortality rate in earthworms depending on the dose. In her paper, “Microplastics in the Terrestrial Ecosystem: Implications for Lumbricus terrestris (Oligochaeta, Lumbricidae),” Huerta Luanga hypothesized that mortality is (partly) due to microplastics abrading the digestive tracts of earthworms. Which make it more difficult for them to absorb nutrients. Damage to the digestive tracts of earthworms that ingested microplastics has also been documented in other articles, as in “Histopathological and molecular effects of microplastics in Eisenia andrei Bouché.” Once microplastics enter an ecosystem, they can proliferate through trophic levels, such as when a bird eats an earthworm. Or when a person eats an apple.
Passing Through Plant—and Human—Tissue
Professor Yongming Luo at the Yantai Institute of Coastal Zone Research and the Nanjing Institute of Soil Science in China, reported microplastics accumulation in “Effective uptake of submicrometre plastics by crop plants via a crack-entry mode” in wheat and lettuce plants exposed to microplastics. The researchers grew the plants in hydroponic and soil systems with microplastics that had been laced with fluorescent dyes. The researchers analyzed cross sections of the plants under a microscope outfitted to detect the fluorescence. The roots, stems, and leaves all lit up.
For decades scientists have believed that plastic particles are too large to pass through the physical barriers of intact plant tissue. But this new study disproves this assumption. The report shows that microplastics appear to enter the plant through cracks in the roots where lateral branching occurs, as well as diffusing through cells at the developing root tips.
A team of scientists also reported in “Micro- and nano-plastics in edible fruit and vegetables. The first diet risks assessment for the general population,” that they had detected microplastics in Italian supermarket produce. Including carrots, lettuce, broccoli, potatoes, apples, and pears. The researchers noted that they found the most microplastics contamination in apples and the least in lettuce. They speculated that the perennial nature of a fruit tree allowed microplastics to accumulate more than in annual crops.
If microplastics are getting into our vegetables, they are getting into everything that eats vegetables, like our meat and dairy. Microplastics have previously been detected in honey, beer, and seafood. With clear and uncontrolled pathways into human food systems, ingestion of microplastics by humans is practically unavoidable.
Documented in “Plastic and Human Health: A Micro Issue?”, plastic microfibers were found in malignant lung tissue biopsies of cancer patients. These plastics were likely inhaled rather than swallowed. But there remains the concern that microplastics can become lodged in tissue and cause dangerous inflammation. Studies of mammals forced to ingest microplastics have also provided evidence that microplastics can pass through cell walls. They can move through the body, accumulate in organs, and impact the immune system.
Microplastics are chemically active materials, capable of attracting and binding to compounds known to harm human health. In addition to cadmium, microplastics have shown to accumulate lead, polychlorinated biphenyls (PCBs), and pesticides. Plastics manufacturing adds its own suite of toxic compounds, including bisphenol-A (BPA), an endocrine disruptor. Researchers have suggested that both acquired and endogenous compounds could leach out of degrading plastics into their environment, whether that be soil or human tissue.
The Plastic Soup Foundation (PSF), a group dedicated to ending plastic pollution, expressed that while we are concerned that microplastics harm our health, precautionary principles have not been applied. As long as there’s no proof, the consensus is that it’s okay that we’re being exposed to these particles every single day, by our food, water, the air we breathe.
What to Do
Since microplastics enter agricultural systems through a variety of means, addressing this issue would require a multi-tiered approach. The PSF has a long standing campaign to eliminate the use of plastic microbeads in personal care products. This would likely reduce the amount of plastic that ends up in sewage sludge. The group also supports limiting single-use plastics generally, as these will ultimately break down to microplastics that end up polluting both ocean and terrestrial environments. While plastic can be very useful for certain purposes, the way we’re using it now is just really not the best long-term approach.
The ECHA proposed an EU-wide ban on intentionally introduced microplastics, including those in personal care products as well as the slow-release fertilizers and seed coatings used in agriculture. Some states in the U.S. (Arizona, Colorado, Connecticut, Hawaii, Illinois, Maryland, Maine, Minnesota, Mississippi, New Jersey, New York, Texas, Virginia, Washington, and Wisconsin), based on concerns by the National Caucus of Environmental Legislators (NCEL) have also moved to ban microbeads from personal care products. As a result of action from 23 states, Congress passed a stronger bill banning the production of microbeads effective July 2017.
One step to address the plastic mulch issue, is to make companies that manufacture plastic mulch films responsible for their recycling and disposal. This would help lift the financial burden and reduce inappropriate disposal at the farm-level.
The use of biodegradable plastics for mulch is a possibility. But these polymers come with their own set of problems. For instance, in “Macro- and micro- plastics in soil-plant system: Effects of plastic mulch film residues on wheat (Triticum aestivum) growth,” biodegradable plastic negatively impacts wheat growth more than a conventional plastic. Also, there is controversy over whether some “biodegradable” plastics actually degrade into harmless compounds, or whether they just break down into microplastics faster. Such controversy surrounds OXO-biodegradable plastics, which the EU moved to ban in 2019.
Another alternative to plastic mulch involves growing nutrient-sequestering cover crops and then rolling them down to form a thick mat. Farmers then plant into the mat, which persists and inhibits weeds, lets water through, and adds nutrients instead of microplastics. According to “Beyond Black Plastic,” this technique, developed at the Rodale Institute in Pennsylvania, can replace more than 90 pounds of plastic mulch per acre. This strategy is scalable to large farms, and emphasizes the importance of building up healthy soils on farmlands in support of long-term resiliency.
More research is needed for a more complete picture of the impacts of microplastic pollution on farmlands. In the meantime, plastics continue to accumulate. In a kind of irreversible contamination. There’s no way to remediate this kind of contamination at the scale of agricultural soils.
Source and photo credits:
There is an Alarming Amount of Microplastics in Farm Soil—and Our Food Supply
https://civileats.com/2021/01/27/there-is-an-alarming-amount-of-microplastics-in-farm-soil-and-our-food-supply/
BY KATE S. PETERSEN, ENVIRONMENTAL HEALTH NEWSJANUARY 27, 2021
Environmental Health News
https://www.ehn.org/
Microplastics in farm soils: A growing concern
https://www.ehn.org/plastic-in-farm-soil-and-food-2647384684/particle-6
Aug 31, 2020