MICROPLASTICS: SOIL POLLUTION RESEARCH PT.2
While all the attention is focused on the impacts microplastics have on our oceans, researchers say that the majority of microplastics are accumulating in soil, including agricultural areas.
An assessment by Environmental Science & Technology suggests that annually, between 107,000 to 730,000 tons of microplastics may be entering agricultural soil in the United States and Europe, compared to 93,000 to 236,000 tons that enter the oceans.
According to Lucas Nizzetto, a research scientist at the Norwegian Institute for Water Research, another concerning issue is intentionally produced microplastics, another source of agricultural soil emissions. These include slow-release fertilizers and plastic clothing designed to protect crops from microorganisms.
A separate report commissioned by the European Commission in 2017 estimates that over 8,000 metric tons of plastic come from slow-release fertilizers and end up in agricultural soils each year (although they state that a percentage of this calculation may not be solely microplastics). In 2019, another report by the European Chemicals Agency listed emissions of 10,000 metric tons for slow-release fertilizers and 500 metric tons for treated seeds annually.
When microplastics enter the soil environment and combine with other organic pollutants, their absorption capacity increases significantly due to the small size of the particles and the large surface area, which play a role in the physical and chemical alteration of the soil and affect the health of the soil ecosystem. Liu et al. (2017) investigated the impact of microplastics on dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved organic phosphorus (DOP), phosphate concentration (PO43), fluorescein diacetate hydrolysis (FDA) and phenol oxidase activities. High concentrations of microplastics had a significant effect on the concentrations of DOC, DON, DOP, humus, and fulvic acid after 30 days of incubation.
De Souza Machado et al. (2019) studied the effects of four commonly used microplastics on soil structure and microbial function. They measured the impact of microplastics on microbial biomass density, water-holding capacity, and the functional relationship between microbial activity and water-stable aggregates in a 5-week experiment on soil cultivation. The findings revealed that different types of microplastics have different effects. For example, polyester could reduce the amount of water-stable aggregates in the soil, while polyethylene significantly increased the amount of water-stable aggregates. The reduction of water-stable aggregates significantly reduces the diversity of the soil microenvironment.
IMPACT ON SOIL WORMS
Currently, soil worms are the most studied organisms in soil ecology. They can transport microplastics from the soil surface to deeper layers, increasing their spread. However, very few studies have been conducted on the molecular and biochemical effects of microplastics on soil fauna.
Cao et al. (2017) suggest in their studies that microplastics significantly inhibit the growth of soil worms at different concentrations of 1% and 2%, posing a highly toxic effect on them. Microplastics enter the bodies of worms after ingestion, causing damage to their intestines, accumulating in their bodies, and affecting their food and secretion, which is closely linked to the survival of soil worms.
Microplastics, specifically Polyethylene (PE), show a clear effect on the histopathological damage and immune system of soil worms, increasing their protein, lipid, and polysaccharide content by 10%. Another study conducted to determine the combined effect of microplastics, polyurethane foam, and PBDEs (polybrominated diphenyl ethers) on soil worms reveals that PBDEs can accumulate in soil worms and thus affect other organisms through the food chain. Consequently, microplastics impact the reproductive rates, growth, and survival of soil worms.
Researcher Esperanza Huerta Lwanga, affiliated with Wageningen University & Research in the Netherlands and El Colegio de la Frontera Sur in Mexico, has studied the effects of microplastics on soil worms. Soil worms are widely known creatures and considered beneficial for agriculture due to their ability to decompose organic matter, enhance nutrient cycling through waste deposition, and improve soil aeration.
"When I was conducting research on soil invertebrates in different backyard gardens in Tabasco, Mexico, I found microplastics, and in these gardens where microplastics were present in the soil, there were no soil worms," says Huerta Lwanga to EHN.
This observation motivated her to study soil worms closely. In her experiments, she discovered that soil worms tended to avoid microplastics, but when the concentration of soil reached 7%, they started ingesting them along with the soil, concentrating the plastics in their castings and transporting them to different soil layers. In 2018, Huerta Lwanga's team warned that rainwater flows through the burrows of soil worms into the subterranean waters, creating a clear pathway for microplastics to enter underground water systems.
She also states that microplastics have increased the mortality rate of soil worms by 8%-25% depending on the ingested dose. In their study, she and her colleagues speculated that mortality might be partially caused by microplastics clogging the digestive tract of the worms, making it harder for them to ingest nutrients. Damage to the digestive tract of soil worms that ingest microplastics has been documented by other researchers as well.