Anaerobic Degradation of [¹⁴C]Methiozolin under Aquatic Sediment Conditions

The fate of methiozolin under anaerobic conditions was investigated in clay loam with a high organic carbon content and sandy loam with a low carbon content using [dihydroisoxazole ring-¹⁴C] and [phenyl-¹⁴C] radiolabels. The sediment/water ratio was 1:3 based on the dry weight:volume (w/v) ratio; the incubations lasted up to 355 days after the treatment (DAT) and were performed in the dark at 20.4 ± 0.7 °C. The overlying water flow-through systems consisted of glass vessels containing sediment with traps for [¹⁴C]carbon dioxide and [¹⁴C]volatiles. The samples were collected and analyzed at 0, 3, 7, 14, 50, 100, 200, and 355 DAT. The water and sediment samples were extracted with solvent systems, centrifuged, concentrated, and analyzed by liquid scintillation counting and a high-performance liquid chromatography (HPLC) system equipped with a flow scintillation analyzer. Following extraction, the sediments were air-dried, and the subsamples were combusted. [¹⁴C]Methiozolin was degraded in the water phase and partitioned rapidly into the sediments, where it was further degraded to other metabolites, which were identified by HPLC and liquid chromatography- or gas chromatography-tandem mass spectrometry (MS/MS) with authentic standards. The dissipation of methiozolin from the overlying water was rapid (with half-lives of 1.1-1.8 and 3.6-4.9 days in the clay loam and sandy loam, respectively). However, methiozolin dissipation from the sediment phase and the whole system was much slower than from the water phase (with half-lives of 122.0-220.0 and 110.0-130.0 days in the sediment phase of the clay loam and sandy loam and 116.0-166.0 and 70.8-85.7 days in the whole system of the clay loam and sandy loam, respectively).