Microplastics appear to be interfering with the very fabric of Earth: its soils and the millions of microbes living in it. In turn, these pervasive plastics could influence soil-carbon cycling and storage, according to a peer-reviewed commentary published in PLOS Biology.
Plastic pollution is a global problem, with plastics big and small piling up in our oceans, on the seafloor and along our beaches. But scientists are only just starting to appreciate the impact microplastics could have on soils – where they are less easily noticed and may be a far greater problem.
Early research suggests that microplastics, which measure less than 5 millimetres in size, may affect microorganisms, including bacterial communities and root-colonising mycorrhizal fungi, and their metabolism.
On this microscopic level, microplastics could thereby disrupt the cycling of nutrients and carbon through soils, and possibly the global carbon cycle at large, if they continue to accumulate in soils as expected.
The problem is, with a slim margin remaining in the global carbon budget to stay below critical limits of global warming, scientists have been counting on soils storing large amounts of the stuff.
And as the review paper outlines, there are numerous ways that microplastics could be having an impact on soils and carbon storage – or at least muddying measurements of soil carbon stocks, as microplastics are themselves made mostly of carbon.
Microplastics in soils might change how fast plants grow, make soils hold more water, change soil structure and stability, interfere with microbial activity, and alter rates of litter decomposition; though the limited and sometimes conflicting research to date suggests the effects could go either way.
Scientists also have no firm idea just how much microplastic has sunk into soils and frankly, quantifying the amount of microplastics in soils could be the hardest part.
“Examining these effects [of microplastics on soils] in detail will be an immense challenge,” the research team writes, “because microplastics come in a dizzying diversity of types … that very likely differ quite drastically in their effects.”
As for soil carbon itself, there are still many unknowns about the natural processes and human practices affecting soil carbon stocks and fluxes, says Dr Samantha Grover, a soil scientist at RMIT University in Melbourne.
“Microplastics is one more thing to throw into the mix,” Grover says.
Soil scientists will need to investigate further and develop new techniques for separating out microplastics from soils before measuring its carbon content, to distinguish between plant-based and plastic carbon sources, says Grover.
Plus, more research looking at the life cycle of microplastics would be useful, she says, to explore how these persistent pollutants are entering urban soils versus agricultural land, where many farmers are looking at ways to manage soils to boost carbon stocks.