Agricultural versus urban soil: Differences in microplastic profiles for a better understanding of the sources and fate
Résumé
Terrestrial soils are a major sink for microplastics, yet they are less well studied than aquatic envrionments. One area of interest is agricultural soil due to the impacts of microplastics on soil structure, crop growth, and toxicology implications for humans and other organisms consuming crops. Agricultural soil is known to have various microplastic sources most notably the application of water/wastewater treatment sludge as fertilizer, as it contains large quantities of polyester fibres from washing machine effluent. Other relevant sources include plastic mulching, netting, greenhouses, plastic drainage pipes, and atmospheric deposition. The transfer dynamics and characteristics of microplastics between different environmental compartments including soil in the same agricultural watershed are not well understood. In this study, soil samples were collected within the Orgeval watershed (104 km2), a long-term French research site composed largely of intensive cereal crops, located near the extremities of the Eastern Parisian suburbs. Composite surface soil samples and soil cores were collected from ten locations within the watershed including upstream and at the outlet. These included soils experiencing various agricultural practices such as gardens, tilled or undisturbed fields, greenhouses, and drainage canal riverbanks, plus soil in forested areas. Additionally, urban soil cores were collected at a separate site 50 km Northwest of Orgeval in a biofiltration swale located along a high-traffic highway. These urban soil cores had an average of 45.0 ± 42.8 (n=9) microplastics per gram of surface soil, their concentration of microplastics decreased with sampling depth, and they consisted mainly of polypropylene. In comparison, early results at the Orgeval agricultural soils contain concentrations of 4 to 5 times lower, including in a greenhouse field with mulching is applied. Polyethylene is the mainly found polymer followed by polypropylene. This comparative study helps better understand the sources of microplastics in soils as well as their fate.