Effect of soil physical and chemical properties on the sorption-desorption hysteresis of the diketonitrile metabolite of isoxaflutole

Isoxaflutole, 4-(2-methanesulfonyl-4-trifluoromethylbenzoyl)-5-cyclopropyl isoxazole] is a relatively new pre-emergence herbicide which undergoes rapid conversion to a diketonitrile metabolite in soil. The half-life of isoxaflutole is very short but the half-life of diketonitrile is much longer and hence, diketonitrile remains for a extended period of time in soil. Sorption-desorption studies were conducted with five soils varying in physical and chemical properties. The batch equilibration technique was used for the sorption experiments, while completely mixed batch reactor systems with the decant and refill method was used for the desorption experiments. Four subsequent desorptions were examined after the sorption process in each soil with an equilibration period of seven days. An apparent sorption-desorption hysteresis was observed in all five soils. Organic matter content and the clay content of the soils were the two determining factors for hysteresis. In soils with high organic matter content, the sorption-desorption hysteresis was mainly governed by organic matter content, but in soils with low organic matter clay content played an important role. With the exception of the Chelsea soil, which had a very high organic matter content (57.4%), all other soils exhibited a high correlation between the clay content and hysteresis index (HI) values calculated at 0.75 ( r ² = 0.960), 25 ( r ² = 0.934) and 150 mg L^?1 ( r ² = 0.928). In conclusion, the potential for leaching through soil and crop injury due to isoxaflutole and its metabolite would decrease as soil organic matter and clay content increases.