Vertical distribution of microplastics in an agricultural soil after long-term treatment with sewage sludge and mineral fertiliser
Environmental Pollution2024
35 citations
?
Citation count from OpenAlex, updated daily. May differ slightly from the publisher's own count.
Score: 65
?
0–100 AI score estimating relevance to the microplastics field. Papers below 30 are filtered from public browse.
After 24 years of use, researchers found that sewage sludge applied as fertilizer deposited significantly more microplastics in farm soil than mineral fertilizer, with substantial amounts migrating deeper than the plow layer -- down to 70 cm. Textile-related plastic fibers were especially common in sludge-treated soil, and smaller fragments moved deeper over time. This shows that decades of applying treated sewage to farmland creates lasting microplastic contamination throughout the soil profile.
Sewage sludge applications release contaminants to agricultural soils, such as potentially toxic metals and microplastics (MPs). However, factors determining the subsequent mobility of MPs in long-term field conditions are poorly understood. This study aimed to understand the vertical distribution of MPs in soils amended with sewage sludge in comparison to conventional mineral fertiliser for 24 years. The depth-dependent MP mass and number concentrations, plastic types, sizes and shapes were compared with the distribution of organic carbon and metals to provide insights into potentially transport-limiting factors. Polyethylene, polypropylene and polystyrene mass concentrations were screened down to 90 cm depth via pyrolysis-gas chromatography/mass spectrometry. MP number concentrations, additional plastic types, sizes, and shapes were analysed down to 40 cm depth using micro-Fourier transform-infrared imaging. Across all depths, MP numbers were twice and mass concentrations 8 times higher when sewage sludge was applied, with a higher share of textile-related plastics, more fibres and on average larger particles than in soil receiving mineral fertiliser. Transport of MPs beyond the plough layer (0-20 cm) is often assumed negligible, but substantial MP numbers (42 %) and mass (52 %) were detected down to 70 cm in sewage sludge-amended soils. The initial mobilization of MPs was shape- and size-dependent, because the fractions of fragmental-shaped and relatively small MPs increased directly below the plough layer, but not at greater depths. The sharp decline of total MP concentrations between 20 and 40 cm depth resembled that of metals and organic matter suggesting similar transport limitations. We hypothesize that the effect of soil management, such as ploughing, on soil compactness and subsequent transport by bioturbation and via macropores drives vertical MP distribution over long time scales. Risk assessment in soils should therefore account for considerable MP displacement to avoid underestimating soil exposure.