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Microplastic risks in soils are fragmentation kinetics- and time-dependent
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Scientists studying plastic-coated fertilizer found that as microplastics break down into smaller pieces over time, they become riskier—not less risky—to soil creatures and plants, because tinier fragments are easier for organisms to accidentally eat or that block plant roots. This matters because it means the danger from microplastics in farm soil actually grows for years after they're applied, so current safety assessments that only look at a single snapshot in time may be underestimating long-term risks to the food we grow.
Abstract Ecological risk assessments for microplastics at steady-state environmental concentrations exist, but these are inherently retrospective and do not account for time-dependent fragmentation processes. As plastics degrade into smaller particles, their exposure, bioaccessibility and consequently their potential risk to biota increase. Here, we present the first prospective, temporally explicit risk assessment framework for fragmenting microplastics, in which exposure assessment is based on particle fragmentation modeling. We apply this framework to the use of polymer-coated fertilizer (PCF) in soils, an agricultural practice with growing demand. The fragmentation model reproduced the PCF concentration data well over the seven-year measurement period of a ten-year field experiment. Food-dilution-based effect thresholds for soil animals and physical-blocking-based effect thresholds for plants, both depending on particle volume, were used to construct species sensitivity distributions (SSDs) and to derive hazardous concentration for 5% of species (HC 5 ). By explicitly resolving fragmentation over time, we show that as particle volume decreases with fragmentation, HC 5 values, and risk characterization ratios vary over time, and identical HC 5 values may correspond to different risk characterization ratios depending on exposure dynamics. Exposure and effects data were aligned by particle size and volume for consistent risk characterization, and assessments were conducted probabilistically to capture uncertainty in fragmentation modeling and data alignment. Risk scenarios for single and repeated PCF applications predict increasing food-dilution risks in animals and physical-blocking-based risks in plants, over 30 years. The magnitude of risk over time depended on both the reapplication frequency and the microplastic fragmentation rate. While demonstrated for PCFs, this modular framework is directly transferable to other fragmenting microplastic sources in soil, such as mulch films and biosolids. The framework developed in this study has high potential to support regulatory evaluations of fragmenting microplastic scenarios in soil systems.
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