Plastic contamination of forest, urban, and agricultural soils: A case study of Yeoju City in the Republic of Korea

Purpose: Microplastics have been widely reported to contaminate aquatic environments, particularly impacting marine organisms, but little is known of microplastic contamination of the soil environment. This study compared the distribution of microplastics in forest, urban, and agricultural soils, investigating the reasons for differences in abundance associated with land use. Materials and methods: We analyzed distribution and abundance of microplastics in 100 soils, representing different land use types that include forest, urban (traffic and residence), and agriculture the environs of Yeoju City. Sampling plots were located on a systematic grid with 2.5 km × 2.5km spacing. Topsoil (0–5 cm) was collected with a hand auger and samples were pretreated by drying, density separation using ZnCl2 solution, organic matter digestion, and decompression filtration. Abundance of microplastics was measured by counting using a digital stereo microscope; microplastics were grouped by shapes (fragment, film, fiber, and sphere) and color (black, red, green, blue, yellow, white, and transparent). Fourier-transform infrared spectroscopy (FT-IR) analysis was used to identify polymer type of the microplastics. Results and discussion: Soils of Yeoju contained an average 700 pieces·kg−1 of microplastics, but this greatly varied with land use types. Roadside soils had more microplastics (1108 pieces kg−1), mostly black styrene-butadiene rubber (SBR) fragments associated with tire dust. Unexpectedly, the largest amount of microplastics was detected not from urban soils but from an upland soil (3440 pieces kg−1). The mean abundance of microplastics in agricultural soil was 664 pieces kg−1, varying with farming types; the highest abundance was at orchard sites, followed by upland, greenhouse, and then paddy field sites. Polyethylene (PE) and polypropylene (PP) were identified from microplastics sampled at upland, greenhouse, and orchard sites, while SBR-derived microplastics were found more widely in all farmland soils, implicating that mulching film usage and farm machinery. Conclusions: Soil microplastic contamination is significant and widespread in urban and agricultural soils of Yeoju, but differs in form and distribution, according to the locality of traffic and farming. Atmospheric fallout to forest soils is quantified and found to be significantly impacted by microplastics. The use of mulching film as a source of PE and PP presents particular concern in terms of the effects of microplastic contamination on soil quality, health, and functionality in agroecosystems