Assessing microplastic mobility from soils amended with sewage sludge under different land use and rainfall scenarios

Sewage sludge applied in agricultural fields as fertilizer is considered one of the major contributors to microplastic (MP) pollution in soil ecosystems. This study assessed the environmental fate of MPs in soils amended with sewage sludge, using experimental units with uncontaminated soil (control), bare soil amended with sewage sludge (sludge), and soil amended with sludge and cultivated with alfalfa (alfalfa), in a greenhouse experiment. The influence of three high rainfall intensities (137 mm/h, 220 mm/h, and 318 mm/h) on MP mobility by runoff and/or infiltration was evaluated. Results showed that MPs in topsoil (0-5 cm) had a limited mobility over the five-month experimental period. In bare soils, 0.32 % of MPs were transported by runoff and 0.008 % by infiltration, indicating that runoff is a more relevant process than infiltration. In soils planted with alfalfa, the amounts mobilized by runoff and infiltration were significantly lower (0.004 % and 0.002 %, respectively) than in the bare soil treatment. This can be attributed to the crop's ability to stabilize the soil and the plant-mediated capture. Despite higher water runoff volumes generated by increasing rainfall intensities, the MP mobilization rates remained unchanged, indicating a limited influence of water volume at high-to-extreme rainfall intensities. Most MPs detected in the sewage sludge consisted of polyester fibres of varying sizes. Larger fibres were more susceptible to mobilization by runoff, while the smaller ones were more trapped into the topsoil. This study underscores the significant capacity of agricultural soils to retain MPs and highlights the role of crops in conditioning MP mobility.