Agricultural plastic mulch increases pesticide persistence in soils

Plastic mulch films are widely used in agriculture, often in combination with pesticide application. While biodegradable plastic mulch films (BDM) are designed for soil incorporation, their influence on pesticide fate remains poorly understood. We investigated the influence of BDM on the degradation kinetics of four widely used pesticides (boscalid, pyraclostrobin, fluopyram and pyraflufen-ethyl) in two agricultural soils under controlled conditions. Pesticides were applied at agronomically relevant doses either directly to soil or adsorbed onto a commercially available polybutylene adipate terephthalate-starch BDM prior to soil incorporation, and pesticides and their relevant metabolites were monitored for 112 days. Adsorption to BDM significantly slowed pesticide degradation for all compounds, increasing degradation half-lifes (DT50) by factors of 2-26 compared with direct soil application. While degradation pathways were not fundamentally altered, the presence of mulch delayed metabolite formation and prolonged pesticide persistence. Rapid degradation of pyraflufen-ethyl led to the accumulation of pyraflufen-ethyl metabolite E-3, a highly persistent multi-halogenated metabolite that remains analytically and regulatorily overlooked. BDM itself showed limited mass loss (12%) over the experimental period, suggesting that pesticide-plastic interactions can persist over environmentally relevant timescales. Overall, this study identifies plastic mulch residues as overlooked modifiers of pesticide persistence and metabolite accumulation in soil, suggesting that plastic-pesticide interactions warrant further consideration in environmental exposure and risk assessment frameworks.