Enhancing Microplastic Transport Research in Agricultural Soils through Fluorescent Particles: A Simplified Method for Detection and Quantification

Preselection of polymer characteristics has been demonstrated to streamline the recovery process in microplastic transport studies, obviating the need for extensive post-examination procedures. Nevertheless, the labor-intensive nature of microplastic extraction and identification remains a significant challenge. This study introduces a novel protocol employing fluorescent microplastic particles for expeditious identification and enumeration without the requirement for extraction, thereby contributing to the cost-effective advancement of microplastic transport research. Size fractionation was utilized to evaluate the protocol's efficacy with respect to 100–500 µm polyethylene (PE) and polylactic acid (PLA) microplastics in soil and sediment matrices, resulting in a substantial 90 to 95% reduction in sample volume post-sieving. Sample assays were conducted under controlled darkroom conditions using a 365 nm excitation wavelength UV lamp, with a digital camera set at 0.2 s, ISO200, and F5.6. Image J analysis ensured meticulous identification, quantification, and characterization of fluorescent microplastics, revealing 95% precision, a 90% F-score, and an 85% recovery rate. Application of the protocol to an agricultural plot-scale case study demonstrated its effectiveness in identifying and quantifying fluorescent microplastic particles in soil samples. This investigation, conducted on five plots (1m x 1m) subjected to rainfall simulations at an intensity of 60 mm h⁻¹, involved the addition of 7.1 g m⁻² of fine (size 125-150 µm) and coarse (size 425-500 μm) fluorescent polyethylene to the topsoil (1 cm). The results indicated a preferential erosion and transport of the microplastics. Overall, our study underscores the utility of fluorescent particles as proxies and their identification through our developed protocol as an effective means of advancing microplastic fate, transport, and deposition research in field and laboratory-scale experiments. Additionally, it highlights that agricultural land susceptible to soil erosion can constitute a significant reservoir of microplastics for aquatic ecosystems.