Dramatic rise in microplastics in soil, with levels increasing by as much as 1,450% just four years after the application of sewage sludge.

A new study from The James Hutton Institute has uncovered a dramatic rise in microplastics in soil, with levels increasing by as much as 1,450% just four years after the application of sewage sludge. Conducted in collaboration with Robert Gordon University (RGU), the study also found that the concentration of microplastics in the soil remained largely unchanged 22 years following the sludge's application.

Microplastics—tiny plastic particles measuring less than 5mm—can either be intentionally produced (as in cosmetics) or break off from larger plastic items. Sewage sludge, a common fertiliser used in agriculture worldwide, contains significant amounts of microplastics, which are released into the soil when applied. While many countries have regulations in place to manage sewage sludge, these laws mainly focus on toxic substances like heavy metals, leaving microplastics largely unregulated.

The research team examined soil samples from Hartwood, North Lanarkshire, which had been part of a UK-wide sewage sludge experiment between 1994 and 2019. This study aimed to investigate the long-term effects of sewage sludge on soil. Over four years, various types of sludge were applied to land plots, which were then maintained as grassland. Soil samples were collected every two years over a span of 25 years and stored in the National Soils Inventory for Scotland at the Hutton Institute.

Thanks to funding from the Macaulay Development Trust, the archived samples allowed researchers to analyse the persistence of microplastics in the soil over an extended period. They found that microplastics rapidly increased and remained in the soil for decades. Additionally, they discovered that different types of plastics degraded at different rates. Microfibers from textiles (the most common microplastic found) and microfilms from packaging broke down over time, whereas larger plastic flakes from containers and bottles did not. The microplastics that did degrade fragmented into smaller particles, including nanoplastics, which present even greater environmental risks.

The study also provided insights into the potential sources of the microplastics found in the soil, based on the specific types of microplastics identified—some of which are rare and found primarily in industrial settings.

Interestingly, researchers observed that textile fibres from the sludge lost their colour over time, although the reason for this is still unclear. It’s possible that the dyes are breaking down, causing the fibres to fade, or the dyes might be leaching into the environment, potentially introducing additional toxic elements into the soil.

Dr. Stuart Ramage, an analytical chemist at the Hutton Institute and lead author of the study, stated, “The persistence of microplastics in agricultural soils over long periods could harm soil health. Understanding how different microplastics behave over time will help us better assess the broader impact of microplastic pollution in soils.”

Dr. Eulyn Pagaling, a senior environmental microbiologist and principal investigator on the study, emphasised the long-term consequences of sewage sludge use: “Our findings underline the need for careful management of sewage sludge to reduce the introduction of microplastics into the soil. We’re now focused on understanding the broader effects of these microplastics and other pollutants on soil ecosystems, including their impact on crop production.”

Professor Kyari Yates from RGU’s School of Pharmacy, Applied Sciences and Public Health, added: “The persistence and abundance of microplastics in the soil, potentially acting as carriers for other pollutants, strengthens the argument within the scientific community for a precautionary approach in using sewage sludge in agriculture. What may be out of sight is not necessarily without risk.”