Prevalence of microplastics in soil increased 15 times in four years – study

The prevalence of microplastics in soil was more than 15 times its initial level after four years of sewage sludge application, a study has found amid concerns over exposure to humans through contaminated crops.

The research also found the number of microplastics in the soil remained relatively unchanged 22 years after application.

Microplastics are pieces of plastic measuring less than 5mm and can be manufactured intentionally, such as in cosmetics, or broken off from larger pieces of plastic.

Sewage sludge is commonly used to fertilise farmland around the world, and contains large amounts of microplastics, which are released into the soil.

Many countries have legislation to manage sewage sludge, but regulations focus on toxicities such as heavy metals and do not cover microplastics.

Researchers for the study, carried out by the James Hutton Institute in collaboration with Robert Gordon University (RGU) in Aberdeen, looked at soil samples from a field in Hartwood, North Lanarkshire, included in a UK-wide sewage sludge experiment between 1994 and 2019.

Investigating the long-term impact of sewage sludge application, scientists applied a variety of sludges to plots of land for four years and maintained the area as grassland.

Soil samples were taken every two years over a 25-year period and archived in the National Soils Inventory for Scotland at the institute.

Archived samples provided researchers with a “unique” opportunity to analyse microplastics in soils over an extended period, which they were able to do with funding from the Macaulay Development Trust.

In addition to discoveries around the quick increase in and longevity of microplastics, scientists found degradation varied between different forms of plastic.

Microfibres from clothing – the most common type – and microfilms from packaging both broke down in the soil, while others, such as flakes from plastic containers and bottles, did not.

But microplastics which did degrade formed smaller microplastics and nanoplastics, posing an additional hazard to the environment.

Researchers were able to determine the potential sources of the sludges used on the soil samples by examining the types of microplastics present, such as uncommon materials only used in industrial locations.

Textile fibres found within the sewage sludge lost their colour over time, however the cause is unknown. It may be that the dyes are degrading, causing the fibres to fade, however it is also possible that dyes are leaching into the environment, which could cause further toxic effects in the soil.

Dr Stuart Ramage, an analytical chemist and lead author of the study, said: “The persistence of microplastics in large numbers in agricultural soils over long periods of time has the potential to damage soil health.

“By understanding how different microplastics behave over time, we can further examine the impact of microplastic pollution in our soil environment.

“Microplastics in soils may present a pathway for exposure to humans through consumption of microplastic-contaminated crops.”

Dr Eulyn Pagaling, a senior environmental microbiologist and principal investigator on the study, said: “Our results highlight the long-term consequences of sewage sludge application and the importance of managing these sludges to mitigate against a major source of microplastics to soils.

“We are now working to see what these microplastics and other pollutants are doing to soil and how this affects ecosystem services we rely on, such as growing crops.”

Professor Kyari Yates, from RGU’s School of Pharmacy, Applied Sciences and Public Health, said: “The abundance and persistence of microplastics which could potentially act as sources of other pollutants to both soil and possibly ground water supports the push within the scientific community to adopt a precautionary approach in the use of sewage sludges in agricultural settings and perhaps out of site is not necessarily without potential risks.”