EFFECT OF MOISTURE, TEMPERATURE, AND BIOLOGICAL ACTIVITY ON THE DEGRADATION OF ISOXAFLUTOLE IN SOIL

Authors: TAYLOR-LOVELL, SARAH - UNIVERSITY OF ILLINOIS; SIMS, GERALD; WAX, LOYD

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2002
Publication Date: N/A
Citation: N/A

Interpretive Summary: Isoxaflutole is a herbicide recently released for grass and broadleaf weed control in corn. Though currently in use, there are few published studies on the environmental behavior of the herbicide. In this experiment, the effects of moisture and temperature on degradation of isoxaflutole were investigated using soil incubated in the laboratory. Isoxaflutole was rapidly converted to an active product in soil. This active form of the herbicide was then further degraded into an inactive material. The half-life of the compound was six-fold higher in air-dry as compared to moist soil. Results of experiments performed over a range of soil temperatures were used to calculate an activation energy (Ea) value for degradation of the herbicide in soil. The Ea value can be used to predict whether a reaction is caused by a chemical or biological mechanism. Activation energy values for isoxaflutole suggested that the first step in degradation was primarily a chemical process. Activation of the herbicide occurred readily in sterile soil, thus confirming a chemical reaction. Little further degradation of the product occurred in sterile soil, suggesting that the second step in degradation was biologically mediated. These findings provide useful relationships between environmental conditions (moisture and temperature) and the processes of activation and subsequent inactivation of isoxaflutole in soil. Such relationships are used in the development of predictive models that guide management practices in order to minimize herbicide inputs and reduce migration of chemicals off the field. The overall impact of this work is to reduce negative effects of agriculture on the environment.

Technical Abstract: The effect of several environmental factors on the dissipation, transformation, and mineralization of isoxaflutole was investigated in laboratory incubations. In the soil, isoxaflutole hydrolyzed to a diketonitrile derivative, which is the active form of the herbicide. The diketonitrile was then metabolized to an inactive benzoic acid derivative and later into two unknown products which were found only in small quantities. Degradation of isoxaflutole was faster in soil maintained at -100 or -1500 kPa compared to air dry soil. At 25 C, the half-lives for isoxaflutole were 9.6, 2.4, and 1.5 days in air dry, -1500 kPa, and -100 kPa moisture regimes, respectively. Isoxaflutole degradation was also directly related to temperature in incubations at 5 C, 15 C, 25 C, and 35 C. Mineralization and transformation increased with increasing temperatures. An activation energy value (Ea) of 67 kJ/mol for isoxaflutole suggested the transformation of isoxaflutole to diketonitrile was primarily a chemical reaction, as opposed to biologically-mediated process. Biological activity was found to have little effect on the conversion of isoxaflutole to its diketonitrile derivative, with half-lives of 1.8 and 1.4 days, in sterile and non-sterile soil, respectively. However, the transformation of diketonitrile to benzoic acid and the production of the unknown products were greatly reduced in the sterile soil, suggesting a biologically-mediated process.