Biodegradable plastic that gets broken down into microplastics may decrease soil nitrogen levels more than conventional ones, which stunts plant growth
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| Some supermarkets now use biodegradable plastic bags (Credit: UrbanImages /Alamy) |
Biodegradable plastics that get broken down into microplastics appear to damage soil and affect plant growth more than conventional ones, at least over the short term.
Global plastic production reached 400 million tonnes in 2022, most of which isn’t recyclable. Large pieces of plastic that end up in the environment can be weathered into smaller and smaller pieces, until they sometimes aren’t visible.
To reduce pollution by these microplastics, some industries have turned to biodegradable plastics, known as bioplastics, but they may sometimes do more harm than good.
A team led by Jie Wang at Jiangnan University in China collected samples of soil containing no visible plastic from a forested area around the university. After removing any debris, stones and vegetation, the researchers dried the soil and measured its pH, organic matter and nutrient levels.
They then added conventional microplastics, made up of the common plastic polyethylene, to some samples and bio-microplastics to others and then attempted to grow wheatgrass. The plant was also grown in unaltered soil to act as a control.
Just 10 days later, the bio-microplastics had significantly reduced key minerals such as nitrogen in the soil, lowered its microbial diversity and stunted plant growth. The polyethylene microplastics had much less of an effect.
It is unclear why bio-microplastics led to this outcome, however, nitrogen is essential for the production of chlorophyll, the green pigment that allows plants to absorb energy from the sun. Reduced nitrogen levels in the soil may therefore directly affect plant growth.
The team also added extra water to some samples to simulate a flood. Bioplastics had an even greater negative effect in these samples, possibly because the excess water encouraged pathogenic bacteria in the soil that affect plant growth.
“This study represents an important step in understanding the impacts of biodegradable plastics and their risks to agricultural production, particularly in wetland ecosystems,” says Samuel Cusworth at ETH Zürich in Switzerland. “Importantly, this study demonstrates that biodegradable plastics are not necessarily a safe and sustainable alternative and their use must be carefully considered.”
Science of The Total Environment
DOI: 10.1016/j.scitotenv.2024.172949
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