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Vertical distribution of microplastics in an agricultural soil after long-term treatment with sewage sludge and mineral fertiliser
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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.
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Microplastic accumulation and transport in agricultural soils with long-term sewage sludge amendments
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This study examined farmland that received sewage sludge applications for 16 years and found that microplastic levels in the topsoil were about five times higher than in untreated fields. Microplastics also migrated deeper into the soil over time, with migration rates 20 times greater in sludge-treated areas. The findings demonstrate that long-term use of sewage sludge as fertilizer is a significant pathway for microplastic accumulation in agricultural soils.
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Researchers investigated plastic dispersal on agricultural fields 30 years after sewage sludge application, finding that macro- and microplastics persisted and migrated both horizontally and vertically in soil, demonstrating the long-term contamination legacy of sludge-based fertilization.
Microplastics in agricultural soils following sewage sludge applications: Evidence from a 25-year study
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This 25-year study -- the longest of its kind -- tracked microplastics in agricultural soils after sewage sludge was applied and found that plastic levels increased by 700-1,400% and remained largely unchanged over two decades. The persistence of these microplastics in farmland is concerning because they can potentially enter crops and the food chain, and the breakdown of colored plastic fibers may release toxic textile dyes into soil.
Impact of sewage sludge application on soil microplastic accumulation and nutrient levels: Analysis of 22 years of data from central UK farmland
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Researchers analyzed a 22-year dataset from 5,323 fields in central UK to examine the relationship between repeated sewage sludge application and microplastic accumulation in agricultural soils, alongside changes in nutrient levels such as nitrogen and phosphorus. They found that microplastic concentrations increased with cumulative sludge applications while nutrients were taken up by crops, raising concerns about long-term plastic accumulation in farmland receiving sludge-derived fertilizers.
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Agricultural soils from 31 fields with different sludge application histories were analyzed for microplastics, finding significant accumulation in fields with repeated sludge applications and a positive correlation between application frequency and plastic particle counts. The study provides direct field evidence that sewage sludge fertilization is a major pathway for microplastic accumulation in agricultural soil.
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