Soil contamination by microplastics in a small French agricultural watershed

Plastics have many beneficial uses in agriculture, but their degradation contributes to diffuse microplastic (MP) contamination of soils, affecting soil structure, biota, and downstream water quality. Their sources, numerous for agricultural soils, are often difficult to identify locally. Despite an awareness of these threats, few global studies outside China have characterized these contaminants in agricultural soil. Among these studies, few have evaluated the fate of microplastics while taking into account the various agricultural practices. This field study focused on characterizing microplastics in French agricultural soil with various land uses in a small watershed, both at the surface and down to a depth of 60 cm, the typical tillage depth. Microplastics concentrations in greenhouse surface soils using plastic mulching were found up to 1.1 · 10⁴ MP/kg, which is significantly higher than other land uses studied (i.e., agricultural crop fields, stream banks, and forests). Further, microplastic concentrations were found to decrease by > 80 % from the top 20 cm of soil to the below 20-60 cm of soil. These findings highlight the need to minimize microplastic sources in agricultural soil, in particular, from greenhouse films and plastic mulching, through policy and materials that reduce UV degradation.