Fate of isoxaflutole and its diketonitrile metabolite in soils as influenced by edaphic factors and soil types
Isoxaflutole is a new pre-emergence herbicide which undergoes rapid conversion to a diketonitrile metabolite (DKN) in soil. In soil the half-life of isoxaflutole is very short, but the half-life of DKN is much longer. Sorption studies were conducted with five soils varying in physical and chemical properties using the batch equilibration technique. Analysis of 14C-ring labeled isoxaflutole and DKN was performed using liquid scintillation counting, and sorption data were fitted to the Freundlich model. Isotherms of isoxaflutole and DKN in all the soils were nonlinear as depicted by the exponent ( n < 1.0), indicating differential distribution of sorption site energies in various soils. Isoxaflutole and DKN sorption increased with an increase in organic matter content of soils and decreased as the soil pH increased from 4.5 to 8.5, which was depicted by the reduction of KF values. Clay content of the soils did not have a high correlation with KF values, while the sorption of isoxaflutole was not influenced by the Ca2+ concentration in the soil solution. There was an increase in the sorption of DKN with an increase in Ca 2+ concentration of the soil solution, while the net sorption constant (Kd) was correlated to the organic matter content of the soils.
A high correlation existed between the desorption coefficient, KFd and the organic matter content of soils, while the clay content did not greatly influence the desorption of DKN. Although the sorption of DKN was generally reversible, a sorption-desorption hysteresis was apparent in all soils. The site energy distribution curves emphasized the fact that DKN binds tightly to soils with higher organic matter content, and a larger proportion of DKN was retained by those soils.
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. When isoxaflutole and DKN were added to soil, the carbonyl stretching frequency decreased as observed in the diffuse reflectance Fourier transformed infrared (DRIFT) spectra. 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 increase.
0768: Environmental science