Influences of land use and depth profile on the characteristics of microplastics in agricultural soils

Terrestrial soils are an environmental compartment in which microplastics are known to accumulate. Compared to the surface water of global oceans, soils contain more microplastics, however they are less well studied to date. In particular, the applications of wastewater and corresponding sludge as fertilizers are a major source of microplastics to agricultural soil, as they include washing machine effluent which is often concentrated in polyester fibres. Other relevant microplastic sources include plastic mulching, netting, greenhouses, plastic drainage pipes, and atmospheric deposition. The characteristics and transfer dynamics of microplastics between different environmental compartments including soil in the same agricultural watershed are not well understood. Additionally, very limited information is known on the stock of microplastics in soils. In this work, a long-term French research site, the Orgeval watershed (104 km2), was sampled for soil. This watershed, located slightly beyond the extremities of the Eastern Parisian suburbs, is composed largely of intensive cereal crops and minimal urban zones. Nine locations within the watershed were composite sampled at the soil surface including locations both upstream and at the watershed outlet. These soil samples were derived from various land use areas including agricultural zones such as tilled or undisturbed agricultural fields, greenhouses, and drainage canal riverbanks, plus soil in forested areas and an urban green space. Of these land use types, greenhouse soils demonstrated the highest concentrations of microplastics in surface soils up to 11,200 MPs/kg, where polyethylene and polypropylene made up the majority of the polymers identified. In comparison, forest soils contained far fewer microplastics up to a concentration of 880 MPs/kg. Soil cores were also collected from two of these sites down to a depth of 60 cm, the typical maximum tilling depth used in this watershed. The most noticeable concentration decrease was observed between soil samples collected at the soil surface versus a further 20 cm below it. This study helps better understand the sources of microplastics as well as their fate in agricultural soils.