Persistence of triazole fungicides in agricultural topsoil is driven by reduced bioaccessibility with aging

Triazole fungicides are widely detected in agricultural soils, yet their long-term persistence remains poorly understood. We evaluated how sorption, bioaccessibility, and degradation change with aging under field conditions. Topsoil was collected over 2 years from four Danish fields with detailed spray records. Tebuconazole, propiconazole, and epoxiconazole were detected 4-19 years after the last spray application, often at nearly constant concentrations. For these aged compounds, soil-water distribution coefficients could not be determined because sorption was too strong. Instead, bioaccessible fractions were quantified using hydroxypropyl-beta-cyclodextrin. Aged triazole fungicides consistently exhibited low bioaccessibility and very slow degradation. In contrast, freshly applied metconazole and prothioconazole-desthio declined rapidly and followed double first-order in parallel kinetics. Metconazole initially showed low sorption and high bioaccessibility but became increasingly inaccessible over the following year. As a result, total concentrations stabilized at a low plateau rather than approaching zero. These results demonstrate that aging reduces bioaccessibility and explains the persistence of triazole fungicides in topsoil. A fraction of applied triazole fungicides may therefore accumulate after repeated use, with implications for long-term soil quality and pesticide risk assessment.