Submitted to: Weed Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/26/2004
Publication Date: 9/10/2004
Citation: Bresnahan, G.A., Koskinen, W.C., Dexter, A.G., Cox, L. 2004. Sorption-desorption of "aged" isoxaflutole and diketonitrile degradate in soil. Weed Research. 44:397-403. Interpretive Summary: The amount of pesticide in soil that is available for movement to the target pest or for movement into surface and ground waters is controlled by the degree of binding of the chemical to the soil and the rate at which it degrades. Binding and degradation of pesticides are usually characterized for pesticides freshly added to soil; however we have shown in previous studies that the length of time the pesticide is in contact with soil, aging, can affect these processes. While there is limited information on binding and degradation of aged pesticide residues, there is less information on bioavailability and degradation of pesticide breakdown products or metabolites. The present study was conducted to determine the changes in distribution between soil bound and solution phases of the main metabolite of the herbicide isoxaflutole, a diketonitrile degradate, with incubation time. We found that while the herbicide metabolite slowly degraded in soil, the remaining chemical became more tightly bound to some soils. Therefore, the aged residues would be less available for movement to surface and ground waters. These results are further evidence that increases in binding of the pesticide to soil during pesticide aging should be taken into account during characterization of the environmental fate of pesticides. Scientist who are developing mathematical models of pesticide degradation and transport in soil need to take these results into account and make their models more flexible for binding characterization, which in turn would make the models more accurate in predicting potential movement of herbicides to surface and ground waters.
Technical Abstract: Isoxaflutole is a relatively new herbicide used for the control of weeds in field corn. The objective of this research was to increase the understanding of the behavior and environmental fate of isoxaflutole and a diketonitrile degradate in soil, particularly to determine the strength of sorption to the soil and the importance effects of aging on sorption. Sorption processes directly or indirectly control the amount of herbicide in solution, thereby directly impacting herbicide bioavailability and the persistence and carryover of herbicides. Total 14C-isoxaflutole residue recovery was affected by both soil type and soil pH. Total 14C recovery from sandy loam soil (SL), pH 7.1 and 8.0, was ~85% at wk 0 and at wk 12; in SL soil, pH 5.7, extraction decreased from ~85% at wk 0 to ~65% at wk 12. The largest decrease in total 14C recovery was in silty clay soil (SiCl), pH 7.4, with recovery of 85% at wk 0 and 52% at wk 12. In SL and SiCl soils, 14C isoxaflutole was found to dissipate rapidly after application to soil, at wk 0 recovery ranged from ~42% to 68%, and at wk 12, recovery had decreased to <10%. Decreases in 14C isoxaflutole residues over time in SL and SiCl soils would be consistent with hydrolysis of isoxaflutole and formation of bound DKN residues in the soil. DKN recovery in SiC and SL soil was 41% to 52% at wk 0. At wk 12 in SL soil, pH 7.1 and 8.0, recoveries were similar, ~40%. However, at wk 12 in SL soil, pH 5.7, DKN recovery decreased to ~28%. DKN recovery in SiCl soil at wk 12 was <10%. Total 14C and 14C DKN extractability decreased over time. Increases in sorption of DKN by the SL pH 5.7 soil and the SiCl soil over time, indicate the DKN degradate is tightly bound to the soil and sorption is affected by soil pH and soil type.