Dissipation of the Herbicide [¹⁴C]Dimethenamid under Anaerobic Conditions in Flooded Soil Microcosms

The objective of this research was to investigate the dissipation of the herbicide dimethenamid under anaerobic redox conditions that may develop in the soil environment. Soil−water biometers were prepared with a saturated soil and made anaerobic by either glucose pretreatment (according to the Environmental Protection Agency registration study for anaerobic fate) or N₂ sparging. Treatments included glucose pretreatment, NO₃^- + SO₄^2- amendment, unamended, and autoclaved. Volatile, aqueous, extractable, and bound (unextractable) ¹⁴C-residues were quantified and characterized. The redox potential decreased over time, and evidence of denitrifying, iron-reducing, sulfate-reducing, and methanogenic conditions was observed, depending on the amendments. Anaerobic degradation of ¹⁴C-dimethenamid occurred in all treatments, and the time observed for 50% disappearance (DT₅₀) was 13−14 days for nonautoclaved treatments. ¹⁴C-metabolites accumulated to up to 20% of applied ¹⁴C. At least two major metabolites were observed in nonautoclaved treatments, whereas only one was observed in autoclaved microcosms. More than 50% of the applied ¹⁴C was eventually incorporated into soil-bound residue.

Keywords: Anaerobic aquatic metabolism; anaerobic herbicide degradation; chloroacetamide; dimethenamid; flooded soil; SAN 582 H