Abundance and Characteristics of Microplastics in Soils with Different Agricultural Practices: Importance of Sources with Internal Origin and Environmental Fate

Abstract Despite intensive use of plastics in agriculture, little is known about the occurrence of microplastics (MPs) in agro-environment. To identify the effect of internal sources and environmental fate on MP appearance in farmland, we examined four soil types with different agricultural practices representing either direct application of polyethylene (PE) film—the inside (GS-in) and outside (GS-out) soils of greenhouse and mulch-film use soils (MS)—or rare use of plastics (rice-paddy soils; RS). MP abundance was 10–7,630 items kg-1, with the order of GS-in, GS-out, RS, and MS. Besides the highest MP abundance, rare PE-sheet (4% of MPs) and increased MP abundance with sizes decreasing were unique for GS-in, implying the presence of internal sources other than PE-film and continued MP accumulation. Contrarily, in other outside soils high PE-sheet appearance (10–31%) indicated substantial input via fragmentation of applied PE-film (for GS-out and MS) or via surface-runoff (for RS) while drastic decline of small-sized MPs in order of MS > RS > GS-out than expected from steady-state fragmentation implied significant removal of MPs by surface runoff and/or infiltration. Our results suggest the importance of internal sources and environmental fate as significant factors affecting the occurrence of MPs in agricultural soils.