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More than 90% of plastic waste ends up in the soil, where it breaks down into microplastics that are invisible to the naked eye. Microplastic pollution of the soil poses a severe threat to soil health as it can harm essential microbial communities and reduce crop yields. The presence of these tiny plastics may also worsen climate change by boosting the production of greenhouse gases, according to a new study in Environmental Science & Technology.

Most previous research focused on one plastic at a time and their effect on soil function and nutrient cycling, but microplastics do not tend to occur in isolation. However, Yi-Fei Wang and Dong Zhu of the Institute of Urban Environment at the Chinese Academy of Sciences, along with their colleagues, decided to study the combined effect of various types of plastics on soil and key functions, such as the nitrogen cycle.

To quantify the problem, the team ran a microcosm experiment in the lab, using soil samples mixed with six different types of plastic, including polyethylene terephthalate (PET) and polyvinyl chloride (PVC). They created four distinct groups with varying levels of plastic, from zero plastics (the control group) to five different types of plastic. After 40 days of incubation, they collected the soil and ran several tests. These included measuring soil properties, such as acidity and key enzyme activities, as well as DNA sequencing to identify bacteria and their associated functional genes.

The team's analysis revealed that increasing microplastic diversity leads to significant shifts in soil health. For example, the plastic mixture considerably raised soil pH (making the soil more alkaline) and increased soil carbon content.

Nitrogen loss

However, one of the most important findings was that microplastic diversity boosted the activity of bacterial genes responsible for denitrification. This is the process by which bacteria convert plant nutrient material into nitrogen gas, which is then released into the atmosphere. It not only makes the soil less fertile, but also releases nitrous oxide, a greenhouse gas that is around 300 times more potent in warming the planet than carbon dioxide. The primary cause of this accelerated nitrogen loss was a family of bacteria known as Rhodocyclaceae.

"Our findings contribute to a deeper understanding of the ecological effects of MP contamination on soil health and nutrient cycling," wrote the researchers in their paper. "More importantly, they underscore the need to incorporate MP diversity into soil management strategies to mitigate nitrogen loss and safeguard soil ecosystem services."

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