Researchers in Scotland sampled soil every two years for 25 years from a field experiment where five different sewage sludges had been applied as fertiliser.
Sludge application increased soil microplastic concentrations by 723% to 1,445%, and the plastic was still present at least 22 years after the last application. An earlier estimate by Nizzetto and colleagues (2016) suggested that European farmland receives 125–850 tonnes of microplastic per million inhabitants each year through sludge.
The finding matters because farmland, not the ocean, may be the largest reservoir of microplastics, with implications for soil health and the food chain.
The field study covers one site. A separate 2023 study by Kedzierski and colleagues, based on 442 sampling sites, estimated the world's agricultural soils hold 1.5–6.6 million tonnes of microplastics, and its authors say better data are needed to refine that figure.