Global concentrations of microplastics in soils – a review

Abstract. Worldwide, microplastics (MPs) have been commonly recognized as a threat to soil ecosystems. Terrestrial soils are widely contaminated by MPs due to the application of sewage sludge and wastewater, plastic mulching, littering, the input of tire wear from roads and atmospheric deposition. Within the last decade, an increasing number of individual studies focused on item counts and masses of MPs in different global soil environments. We reviewed these studies to achieve a representative picture of common degrees of contamination. The majority of the prospected agricultural and horticultural sites were exposed to sewage sludge and mulching film application and showed concentrations of <13 000 items kg−1 dry soil and 4.5 mg kg−1 dry soil. Microplastic concentrations in soils in the vicinity of municipal areas were thereby 10 times larger compared to rural sites. The measurement of masses was generally underrepresented compared to item numbers, and mass data were often generated from microscopic analyses by using shape-to-mass models instead of direct measurements. Extreme values, such as on industrial sites, exceed the common concentrations by 2 to 4 orders of magnitude, which might be attributed not only to the land use but also to the applied methods of measurement. Campaigns that focus on other entry pathways like composts, road dust runoff and littering or land uses like grassland, forest, fallow and wilderness as well as industrial sites and landfills were underrepresented or nonexistent. Background loads, such as atmospheric deposition, were often not excluded from the measurements and, thus, the studies might overestimate the contribution of the analyzed entry pathway to the total load. Other studies focused on light density MP, e.g., from mulching films, and therefore underestimated the amount of soil MP. Despite these limitations, the data give an impression of the spectrum of global MP concentrations and are a good basis for experiments examining the effects of MPs on exposed soils. Based on the collected data, we identified problems of past studies and recommend that future experimentation take into account standardized methods of extraction and quantification, a proper characterization of the sampling sites and their history as well as the exploration of as yet underrepresented entry pathways and land uses.