Transport of Polylactic Acid (PLA) Microplastics in Agricultural Soils under Simulated Rainfall Events - Analyzing Surface Water Runoff as an Environmental Pathway

Due to the ecological risk posed by plastic pollution, biodegradable plastics (BDPs) usage in agriculture has gained attention as an alternative to conventional plastics. However, agricultural soil remains a major reservoir of microplastics (MPs) in today’s global environment. Given the persistence of MPs, few studies have focused on the fate and transport of conventional MPs from agricultural topsoil. However, understanding the transport of biodegradable MPs is largely unknown. The aim of this study is to analyze the erosion and transport behavior of biodegradable polylactic acid (PLA) MPs (size 250 – 300 μm) under simulated heavy rainfall events on fallow and crusted agricultural soils. The experiment used fluorescent PLA filament to obtain the MPs. Based on the fluorescent property of the PLA MPs, a simple, reliable, and cost-effective method using darkroom photography under a 365nm UV lamp, and an Image J script was developed for quantification. The study was conducted on three plots (1m × 1m) of loamy soil at an agricultural field in the Czech Republic. A concentration of 4 g m^-2 of PLA MPs was incorporated into the top 5 cm of the soil in each plot. Rainfall simulations with an intensity of 60 mm h^-1 were carried out twice within one week in April 2022. The results showed a general depletion of biodegradable PLA MPs in the delivered sediment. Eroded sediments showed mean enrichment ratios of 0.59 ± 0.05 in the fallow plot and 1.45 ± 0.37 in the crusted plot. Higher MP enrichment and sediment delivery were observed on crusted soil compared to fallow soil, suggesting a more conservative erosion and transport for the latter. Overall, our study highlights that a relatively high-density biodegradable MP, such as PLA, tends to remain within the topsoil and is negligibly affected by rainfall-induced surface runoff and very low infiltration capacity.