Environmental fate and risk of diverse oxygenated isomeric prothioconazole biotransformation products in soil

Isomeric biotransformation products (TPs) of organic pesticides substantially expand structural diversity and may exhibit distinct environmental behaviors compared with their parent compounds. However, the formation mechanisms and ecological risks of such isomers (regioisomers) in soil remain poorly understood. In this study, the triazole fungicide prothioconazole (PTC) was incubated in agricultural soil, and 21 isomeric TPs were identified and classified into six structural groups using high-resolution mass spectrometry (HR-MS) based on diagnostic fragmentation patterns. These secondary oxygenated isomeric TPs were derived from three primary intermediates (PTC-desthio, PTC-dehydrated, and PTC-S-methyl) via site-selective oxygenation. Toxicity predictions indicated substantial variability among isomers, with certain hydroxylated derivatives exhibiting higher predicted ecotoxicity than others, challenging the assumption that biotransformation necessarily leads to detoxification. Metagenomic analysis showed that PTC exposure significantly altered the soil microbial community, enriching genes associated with xenobiotic degradation and central carbon metabolism. Notably, functional microbial shifts, particularly involving the genus Acinetobacter, were strongly associated with specific isomer profiles. These results suggest a potential link between microbial metabolic responses and the formation of structurally diverse isomeric TPs. The widespread occurrence of such isomers highlights that assessments based solely on parent compounds may underestimate the environmental fate and risks of PTC in agroecosystems.