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.