The Earth is slowly becoming a planet of plastic. Microplastics have been recognized as an emerging contaminant worldwide, and scientists are only beginning to understand their impacts on the environment. Microplastics can leach into soil systems and be ingested by soil fauna as well as potentially affect human health by entering the food chain. Around 107,000 – 730,000 tons of microplastics could be dumped onto agricultural soils in Europe and the US annually, which is lower compared to the 93,000 to 236,000 tons entering oceans (UNEP, 2018).
How do microplastics reach soil?
There are several pathways through which microplastics enter soils, including through processed sewage sludge and plastic films used in agricultural production. Microplastics from sources like clothing, personal care products (such as microbeads), and urban runoff persist in sewage sludge. When the sludge is applied to agricultural fields as fertilizers, it may lead to thousands of tons of microplastics in soils. Plastic mulches are sheets of plastic laid on soil for weed management to warm soil and retain moisture. These can break down into microplastics in soils. Slow-release fertilizers and plastic-covered seed coatings are other sources of microplastics.
Plastic mulch. Source: Gardening Know How
Effects of microplastics on soils and plants
Lin et al. (2020) was the first field study exploring how microplastics can affect soil fauna. The researchers determined that microplastics reduced the abundance of soil fauna including soil microarthropods and nematodes such as mites, roundworms, and springtails which play important ecosystem functions such as breaking down organic matter. The study concluded that this could have a cascading effect through soil food webs, leading to potential consequences on soil carbon and nutrient cycling. In another experiment on the effect of microplastics on earthworms, earthworms generally avoided microplastics but when the concentration of microplastics reached 7%, they began to accumulate them in their bodies, concentrating plastics in their castings. If rainwater washed through earthworm burrows, it could transfer microplastics into groundwater.
A recent review by Jenkins et al. (2021) also found that microplastic additions to soil generally negatively impact soil physical parameters such as decreasing aggregate stability, bulk density, and micropore volume. Microplastics are capable of absorbing other harmful compounds such as lead, cadmium, and pesticides which also pose a risk to soil and human health.
Source: rePurpose
From soil to spoon?
Microplastics can enter from the soil to plant tissue. For example, Li et al. (2020) found that lettuce and wheat plants that were exposed to microplastics through soil and hydroponics lead to the accumulation of microplastics in plant tissue. Microplastics were also found in supermarket produce in Italy, as they determined that perennial fruits were likely to accumulate more microplastics than annual crops. In Singapore, drinking water is free from microplastics due to water filtration systems, but individuals could be ingesting microplastics in their food depending on one's diet. WWF released a campaign called "Your Plastic Diet" which includes a quiz you can take to find out how much microplastics you are ingesting per week, depending on your diet - do try it out for yourself! WWF's study found an average person could be ingesting a credit card (around 5g) worth of microplastics per week.
Although microplastics have been found in human bodies and excretion, it is difficult to conclude whether microplastics pose a significant health risk as scientific research about microplastics is still evolving. At high concentrations and through direct contact, however, they can be toxic. In addition, plastic microfibers have been found in malignant lung tissue biopsies of cancer patients. Although this is likely a result of inhalation rather than food consumption, the result highlights that microplastics can potentially lead to inflammation.
This raises the question – in the absence of scientific certainty of whether microplastics harm human health, should the precautionary principle be applied? Admittedly, plastics have brought the world prosperity, convenience and medical advancements but the way we use them has become highly unsustainable. As the amount of microplastics increases in our environment and enters our food chain, they can accumulate in our bodies and pose bigger risks to human health. Policy changes that prevent microplastic production could be steps in the right direction to save our soils and health. For example, mandating bans on microplastics such as microbeads, seed coatings and fertilizers.
References
It’s on our plates and in our Poo, but are microplastics a health risk? (2021, May 26). the Guardian. https://www.theguardian.com/environment/2021/may/15/its-on-our-plates-and-in-our-poo-but-are-microplastics-a-health-risk#:~:text=Studies%20have%20found%20microplastics%20%E2%80%93%20that,Bottled%20water%20contains%20even%20more
Lin, D., Yang, G., Dou, P., Qian, S., Zhao, L., Yang, Y., & Fanin, N. (2020). Microplastics negatively affect soil fauna but stimulate microbial activity: Insights from a field-based microplastic addition experiment. Proceedings of the Royal Society B: Biological Sciences, 287(1934), 20201268. https://doi.org/10.1098/rspb.2020.1268
Mbachu, O., Jenkins, G., Kaparaju, P., & Pratt, C. (2021). The rise of artificial soil carbon inputs: Reviewing microplastic pollution effects in the soil environment. Science of The Total Environment, 780, 146569. https://doi.org/10.1016/j.scitotenv.2021.146569
Petersen, K. S. (2021, March 8). Microplastics in farm soils: A growing concern. EHN. Retrieved February 5, 2022, from https://www.ehn.org/plastic-in-farm-soil-and-food-2647384684/particle-11
Plastic planet: How tiny plastic particles are polluting our soil. (2018, April 10). UN Environment. https://www.unep.org/news-and-stories/story/plastic-planet-how-tiny-plastic-particles-are-polluting-our-soil
Watts, J. (2020, September 2). Microplastic pollution devastating soil species, study finds. The Guardian. https://www.theguardian.com/environment/2020/sep/02/microplastic-pollution-devastating-soil-species-study-finds