Relationship between herbicide concentration and the rates of enzymatic degradation of¹⁴C-diallate and¹⁴C-triallate in soil

Abstract

The influence of concentration on the rates, routes and patterns of loss of [carbonyl-¹⁴C] diallate and [allyl-2-¹⁴C] triallate from an agricultural soil was studied in the laboratory for up to 52 weeks using an incubation system that allowed 95 to 98% recoveries of the added¹⁴C. Based on comparison of the half-lives, diallate was dissipated from soil at three to four times the rate of triallate at all five concentrations (0.25 to 50 μg/g) tested. The major routes of loss, in descending order of importance, were degradation, bound residue formation, and volatilization. With both herbicides,¹⁴CO₂ was the only degradation product identified; however, large quantities of bound residue and traces of benzene- and water-soluble radioactivity were also detected. Evidence for the biodegradability of the bound residue of diallate was given. Although dissipation rates could not be described by zero-, half-, first-, or second-order kinetics, a reaction order between first- and second- was indicated. Enzymatic material in the soil had the potential to degrade at least one μg/g diallate within 24 hr and 0.5 μg/g triallate within 72 hr. However, when these quantities of herbicide were applied as initial dosages to the soil, measurable amounts of the parent molecules could be detected after 30 (diallate) and 52 (triallate) weeks of incubation. It was concluded that a primary factor influencing the rates of enzymatic degradation of herbicides in the soil is a physical separation of the herbicide molecules and the enzymatic systems responsible for degradation.