Omar Khalid
Plastic is a versatile material that has improved our lives in many ways. However, it has also become a significant environmental concern due to its persistence in the natural world. Plastic pollution is infiltrating the world's soils, particularly agricultural soils, through various sources, including sewage sludge, coated fertilizers, irrigation water, and agrochemicals. This contamination can lead to the breakdown of plastic into microplastics, which can have harmful effects on the soil ecosystem and potentially enter the food chain. While biodegradable plastics have been developed, the challenge of managing and removing plastic from soils remains. The long-term impact of plastic breakdown in soils is still uncertain, and researchers are working to develop standardized methods for detecting and understanding the presence of microplastics in soil.
| Characteristics | Values |
|---|---|
| Plastic breakdown in soil | Plastic can take anywhere from 20 to 500 years to decompose, depending on the material and structure |
| Factors influencing breakdown | Sunlight exposure, moisture, acidity, ultraviolet light, and the size and type of plastic |
| Impact on soil | Reduced soil quality, potential long-term damage, and entry into the food chain |
| Detection methods | Hyperspectral imaging, floatation methods, laser-induced breakdown spectroscopy |
| Mitigation strategies | Biodegradable plastic, plant-based plastics, modified chemical bonds for easier breakdown |
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What You'll Learn
Plastic contamination in agricultural soils
Agricultural soils, including arable lands, paddy fields, uplands, irrigation areas, and greenhouse soils, are all affected by plastic contaminants. These contaminants come in various sizes, from macroplastics (≥ 5 mm in diameter) to mesoplastics (5 mm–2 cm), microplastics (< 5 mm in diameter), and nanoparticles (< 1 μm). The primary sources of these contaminants include sewage sludge, coated fertilisers, irrigation water, and agrochemicals. Secondary sources include the breakdown of larger plastic materials used in agricultural practices, such as mulching and greenhouse films.
The extensive use of plastics in agriculture, particularly plastic mulch films, has led to widespread soil contamination. Plastic mulch films cover over 25 million acres of farmland globally, introducing approximately 6.7 million tons of non-biodegradable material into terrestrial ecosystems annually. While plastic mulches are valuable for weed management and temperature and moisture control, they contribute significantly to soil pollution.
The impact of plastic contamination on soil health is concerning. Studies have shown that even low levels of macroplastic fragment accumulation can lead to declines in soil quality. Macroplastic contamination negatively affects soil moisture, microbial activity, available phosphate, and soil carbon pool size. These effects have been observed in farms following "`best practice`" plastic mulching application and removal procedures, indicating that improved management practices are necessary to mitigate the threat to soil health.
Addressing plastic contamination in agricultural soils requires a multifaceted approach. Researchers are working to identify the best methods for studying microplastics and proposing future research areas. Standardised extraction methods and soil characterisation techniques are being developed to better understand the abundance and effects of plastics in agricultural soils. Additionally, there is a growing focus on sustainable alternatives to plastic use in agriculture and the implementation of effective waste management practices.
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Factors affecting plastic breakdown
Plastic breakdown in soil is a complex process influenced by various factors that determine the rate and extent of degradation. Here are some key factors that affect the breakdown of plastic in soil:
Environmental Conditions: Climatic variables such as temperature, solar radiation, precipitation, and wind play a significant role in the physical weathering and aging of plastics. Higher temperatures, sunlight exposure, and wind can accelerate the breakdown process by causing the plastic to degrade and fragment into smaller pieces.
Agrochemicals: The presence of certain agrochemicals, such as pesticides, can induce early aging and degradation of plastic films. Specifically, chemicals containing sulfur, halogen, iron, and chlorine have been found to contribute to plastic breakdown.
Environmental Pollution: Environmental pollutants like hydrocarbons, nitrogen oxides, sulfur oxides, and particulate matter can enhance plastic degradation. These pollutants can abstract hydrogen from the polymer chains, weakening the structure and making it more susceptible to breakdown.
Soil Texture and Structure: The texture and structure of soil can influence the breakdown of plastic. Plastic fragments may introduce fracture points within soil aggregates, increasing the number and size of soil pores. This can facilitate root growth but also lead to increased water loss, impacting soil water content and plant performance.
Microorganisms: Some enzymes produced by microorganisms in the soil may contribute to plastic degradation. However, the specific mechanisms and extent of their involvement are not yet fully understood and require further research.
Type of Plastic: Different types of plastics have varying degradation rates. Compostable plastics, such as blended thermoplastic starches, can degrade within months, while other polymers can take significantly longer to break down. The chemical nature of the plastic plays a crucial role in determining its lifespan in the soil environment.
It is important to note that the breakdown of plastic in soil can have both positive and negative consequences. While it can facilitate plant growth by increasing root space, it also contributes to the widespread presence of microplastics in the environment, which can have detrimental effects on ecosystems and potentially human health.
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Plastic's impact on soil ecosystems
Plastic is everywhere, and it is having a detrimental impact on our soils. The vast majority of plastic waste is not recycled or incinerated, instead ending up in landfills, where it can take up to 1000 years to degrade. As plastic breaks down, it can release potentially toxic chemicals, which can seep into the soil and water, causing a range of harmful effects on the species that ingest them.
The impact of microplastics in soils, sediments, and freshwater could have a long-term negative effect on ecosystems. Terrestrial microplastic pollution is much higher than marine microplastic pollution, and fragments of plastic are present almost everywhere. The concentration of microplastics in the environment is increasing, and it is likely that the concentration in humans is doing the same.
The Food and Agriculture Organization (FAO) and the World Health Organization, among others, organized the Global Symposium on Soil Pollution in 2018 to address the issue of soil pollution. The outcome document, 'Be the solution to soil pollution', led to a coordinated set of actions to #StopSoilPollution.
The contamination of agricultural soils is ubiquitous, and plastic pollution in these soils is receiving increasing attention from scientists and stakeholders worldwide. The distribution of microplastic contamination studies shows that 60% have been conducted in Asia, 29% in Europe, 4% in Africa, and 4% in North America.
The presence of microplastics as pollutants has detrimental effects on soil, surface, and underground water resources, threatening human health. The concentration of microplastics in the soil depends on the quantity and duration of sewage sludge application, with higher pollution levels arising from higher application rates.
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Biodegradable plastics
Plastic pollution is a pressing issue, and one solution that has been proposed is the use of biodegradable plastics. Biodegradable plastics are plastics that can be decomposed by living organisms, usually microbes, into water, carbon dioxide, and biomass. However, it is important to note that not all plastics labelled as "biodegradable" are truly effective in breaking down. Some of these plastics only break down into smaller pieces like microplastics, which can still be harmful to the environment.
There are two main classes of biodegradable plastics: bioplastics, which are derived from renewable raw materials such as plants, animals, or microorganisms; and plastics made from petrochemicals with biodegradable additives that enhance biodegradation. Bioplastics can be produced from a variety of sources, including plants, bacteria, seaweed, and plant waste. One example of a biodegradable plastic is polyhydroxyalkanoate (PHA), which is a class of biodegradable plastic naturally produced by various microorganisms.
The use of biodegradable plastics has been financially viable only in contexts where specific regulations limit the use of conventional plastics. For example, in Italy, biodegradable plastic bags have been compulsory since 2011. Biodegradable plastics are commonly used for disposable items such as packaging, cutlery, and food service containers. They can be foamed into packing materials, extruded, and injection-moulded using modified conventional machines.
While biodegradable plastics offer an ideal solution for single- or short-term use applications, there are challenges to their effectiveness. Many biodegradable plastics are designed to degrade in industrial composting systems, which require well-managed waste systems. If these plastics are discarded into conventional waste streams or end up in the open environment, they may not break down properly. Additionally, the presence of microplastics in the soil, regardless of their origin, is a growing concern.
Scientists are working on improving the biodegradability of plastics. For example, researchers at the University of California, Berkeley, have developed a way to make compostable plastics break down more easily with just heat and water within a few weeks. This process eliminates microplastics and has the potential to revolutionise the recycling industry.
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Microplastics in the food chain
Plastic pollution is a pressing issue that has emerged since the 1950s, with millions of tonnes of plastic waste now present in the environment. Plastic is used in a wide range of applications, from packaging materials to clothing, and its persistence means that it does not easily degrade. This has led to the widespread contamination of the land, sea, and air with microplastics, which have been detected throughout the human food chain.
Microplastics are plastic particles smaller than five millimetres in length. They can be primary, intentionally manufactured small plastic particles such as microbeads in cosmetics, or secondary, formed from the breakdown of larger plastic items through weathering and environmental exposure. These microplastics are pervasive in the environment, and their presence in the food chain is a growing concern.
Aquatic ecosystems, in particular, have been heavily impacted by microplastics. Marine life, from fish to shellfish, ingest these particles, which then accumulate in their tissues. A 2022 study found microplastics in blue mussels off the Australian coast, reinforcing the idea that consuming mussels means consuming microplastics. Microplastics have also been detected in seafood, with a study in Oregon finding them in 98.9% of samples.
It is not just marine life that is affected. Microplastics have been found in a range of foodstuffs, including honey, tea, sugar, fruit, and vegetables. They can reach farmland through sewage sludge used as fertiliser and then enter waterways through runoff from the top layer of soil. This means that even land-based organisms are at risk of ingesting microplastics, which can have adverse health effects.
The health impacts of microplastics are still being studied, but they are known to carry toxic chemicals and microorganisms. They have been linked to a range of issues in humans, including gastrointestinal disorders, respiratory problems, cancer, infertility, and neurotoxicity. Their small size allows them to spread throughout the body, potentially reaching organs like the brain. With microplastics present in the environment and the food chain, human exposure is inevitable, and the potential consequences for health are concerning.
Addressing this issue requires concerted action from businesses, governments, and civil society. While plastic has beneficial properties that have led to its widespread use, the contamination of the food chain with microplastics poses a significant threat to public health and food security. Standardised procedures for the collection and analysis of microplastics are needed, along with strict regulations to control plastic use and ensure food safety.
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Frequently asked questions
Plastic does break down in soil, but it is not easily biodegradable and can last for prolonged periods of time. The lifetime of plastic in soil depends on its chemical nature, the environment, moisture, acidity, ultraviolet light, and the size and type of plastic.
Plastic can take anywhere from 20 to 500 years to decompose, depending on the material and structure. Single-use plastic grocery bags take about two decades to break down, while plastic water bottles made with polyethylene terephthalate (PET) can take approximately 450 years.
As plastic degrades, it can leak toxins into the soil, causing soil contamination and reducing its quality. Plastic can also enter the food chain, with studies identifying microplastics in human organs, including the brain. The long-term effects of microplastics on human health require further research.
