Soil health is a critical global issue. World Soil Day reminds us that degraded soils affect food security, water quality and ecosystems. Among the threats to soil ecology, micro-nano-plastics or MNPs have emerged as a significant new one. Studies suggest that the presence of MNPs in soil hampers seed germination and seedling growth. The vital physiological processes of plants are disturbed, impacting the yield and quality of produce. Plants’ responses to microplastic pollution have been observed through reduced rate of photosynthesis, stunted growth, decreased fruit yield and low nutritional content of fruits and vegetables.

The journey of plastics is long and arduous.

More than 200 million tonnes of plastic waste generated by humans ends up in soil or water, where it begins its centuries-long journey of weathering and degradation, producing micron-sized or nano-scale forms of plastic polymer called micro-nano-plastics or MNPs.

Micro-nano-plastics (MNPs) have emerged as novel environmental pollutants. Their presence in the environment is harmful to human health, leads to ecosystem degradation and poses a threat to soil ecology.

Plastic-contaminated soil is a significant reservoir of MNPs from where they leach into other regions in the soil, enter the food chain when animals eat contaminated plants, and make their way into the human body via food, water or direct contact through skin.

MNPs can be categorised on the basis of size as nano-plastics ranging from 1nm to 100 nm and microplastics ranging from 0.1µm to 1000 µm. Based on their origin, MNPs can be primary or secondary. Primary MNPs are made intentionally in small sizes for products such as exfoliating scrubs, toothpaste or synthetic fibers in clothes. Secondary MNPs form when larger pieces of plastic break down over time due to mechanical, chemical or biological processes.

Effects on soil health

Plastic waste disposed in landfills and garbage dumping sites and plastic pollutants from municipal wastewater and industrial effluents directly add the MNPs in soil and water.

Modern farming practices such as using mulching films, irrigating with wastewater, using polymer coated slow-release fertilizers and applying sewage sludge in agriculture are also responsible for microplastic contamination in agricultural soils.

MNP contamination directly impacts soil health by disrupting its structural integrity and altering its physico-chemical properties such as moisture content, acidity, water holding capacity and soil aggregation or the clumping of soil particles, which influences soil fertility and stability. These changes harm crop growth and agricultural productivity.

MNPs also affect soil microbial diversity which in turn impacts the root ecosystem by reducing soil fertility, plant-microbe interactions, disrupting the movement of nutrients from the environment to living organisms in the soil, and reducing the soil’s ability to absorb nutrients.

MNP residues also interact with other contaminants in the soil and increase its toxicity. For example, polyethylene particles in soil tend to adsorb pesticides from the soil and increase their concentration and toxicity.

Effects on plant and human health

The toxic effects of MNPs do not remain restricted to the soil environment but extend to plant health and human health. Plants take up MNPs from plastic polluted soil through their roots, from where they are transported to the shoot parts of plants such as leaves and fruits. MNPs are further passed on to humans when fruits and vegetables contaminated with MNPs are eaten.

The consumption of MNP-contaminated plant-based food directly exposes humans to serious health risks. Research data reveals the presence of MNPs in edible parts of plants and indicates the hazards to human health. MNPs have been reported in apple, pear, carrot, lettuce, onion, tomato, cucumber and many other commonly consumed fruits and vegetables.

The exact quantification of human consumption of MNPs derived from foods is difficult. However, the potential risks on human health cannot be ignored.

Recent studies report the presence of MNPs in the human body, not only in the tissues, organs and blood, but also in the carotid plaque, linking it directly to cardiovascular diseases leading to heart failure.

MNPs have also been found in human breast milk, raising a serious concern for infants’ health.

As human exposure to MNPs seems inevitable, it is imperative to understand their toxicological effects on human health. Research-based studies using various experimental models such as in-vitro cell and tissue culture, mice models and in-vivo studies in various animals including mammals, birds and fishes suggest the hazardous effects of MNPs on organisms.

MNPs have been implicated in metabolic disorders, endocrine disruption, immunological abnormalities, neurotoxicity and organ dysfunction.

Exposure to MNPs have been found to change reproductive behaviour and interfere in the normal growth and development in many animals.

The presence of MNPs in human specimens such as blood, saliva, stool, placenta and breast milk can potentially pose serious risks such as cancer, gastrointestinal diseases, cardiovascular problems, endocrine conditions, metabolic disorders, reproductive problems, and prenatal and postnatal developmental hazards.

The way forward

The lack of consensus among countries on measures to curb plastics pollution is a serious setback.

MNPs are invisible foes for both humans and the environment. A delayed response in fighting microplastic pollution can potentially cost the health and well-being of the entire ecosystem.

The 3R formula — Reduce, Reuse and Recycle — is no longer sufficient to deal with the situation. More vigorous strategies are required for mitigation of MNP pollution from the environment.

A clearer picture of the detrimental effects of MNPs and creating awareness regarding its potential health risks is important to safeguard public health. More epidemiological research is required focusing on chronic exposure of MNPs and their toxicological effects to prevent high exposure risks to soil and human health.

Dr. Abhilasha Shourie is Professor at the Department of Biotechnology, School of Engineering & Technology, Manav Rachna International Institute of Research and Studies, Faridabad, India.

Originally published under Creative Commons by 360info™.

(Photo Credit: Microplastic contamination directly impacts soil health by altering the properties of soil, with detrimental effects on crop growth and agricultural productivity. Lisa Fotios Credits Pexels)

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