Sorption to mulch film decreases bioavailability of two model pesticides for earthworms in soil

Microplastic pollution and its environmental consequences have been a research topic for decades. Alongside sewage sludge and compost, mulch films are one of the most important sources of organic matter in agricultural soils. As microplastics accumulate in these environments, they increasingly interact with other agrochemical contaminants, such as pesticides. The consequences of the combined presence of mulch film microplastic and pesticides, especially on bioavailability and toxicity, still need to be understood. We investigated the interaction of pesticides with polyethylene mulch films by choosing the neonicotinoid Thiacloprid and the fungicide Tebuconazole in their commercial formulations as plant protection products, serving as representative hydrophobic and hydrophilic substances. The impact of PE microplastic (150 mg/kg) on both pesticides' extractability and earthworm mortality was determined. The hypothesis is that microplastics reduce pesticide bioavailability in soil. Hence, reduced earthworm toxicity might be observed. Acute toxicity testing with Eisenia fetida showed a slight decrease in toxicity while MPP was present, shifting the LC50 from 110.3 to 120.6 mg/kg (Tebuconazole) and 20.3-22.6 mg/kg (Thiacloprid). The chemical analysis after two-time extraction of test soil with CaCl2 and then solvents confirmed the reduced availability of pesticide in the presence of MPP. Our research shows that microplastic at application-relevant concentrations can retain pesticides and reduce pesticide toxicity. These findings impact pesticide efficiency in soils that accumulate microplastic, potentially requiring the adaptation of agricultural practices. This study underscores the need for further research and mitigation strategies as our understanding of the interactions between pesticides and microplastics in soil evolves.