SORPTION OF ISOXAFLUTOLE DIKETONITRILE DEGRADATE (DKN) AND DICAMBA SORPTION IN UNSATURATED SOIL

Authors: Koskinen, William; Ochsner, Tyson; STEPHENS, BRANDON - UNIVERSITY OF MINNESOTA; KOOKANA, RAI - CSIRO LAND & WATER

Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2006
Publication Date: 9/7/2006
Citation: Koskinen, W.C., Ochsner, T.E., Stephens, B.M., Kookana, R.S. 2006. Sorption of isoxaflutole diketonitrile degradate (DKN) and dicamba sorption in unsaturated soil. Journal of Environmental Science and Health. 41:1071-1083.

Interpretive Summary: When characterizing the interactions of weakly-binding or readily degraded herbicides, the normally used batch slurry equilibration methods tend to yield a high margin of error attributable to errors in concentration measurement and degradation, respectively. This study employs a recently developed horizontal flow method using field moist soils to determine the sorption of herbicide isoxaflutole's biologically active breakdown product, DKN, and the herbicide dicamba. DKN and dicamba binding to soils was significantly greater using the flow method as compared to the batch equilibration method in clay loam, but similar for the two methods in sandy soils. This demonstrates that to determine the interaction and binding of degradable and weakly binding herbicides, the flow method with field moist soils may be a suitable alternative to the unrealistic batch slurry method. In fact, it may be better in cases where transport models have over predicted herbicide leaching when batch sorption coefficients have been used. Another advantage of this method is that other physical parameter needed for transport models to predict herbicide movement in soil are readily obtained. Scientists now have a simple laboratory experimental method to characterize herbicide-soil interactions, the results of which may help to explain why many models over predict herbicide movement in soil.

Technical Abstract: When analyzing the sorption characteristics of weakly-sorbing or labile pesticides, batch methods tend to yield a high margin of error attributable to errors in concentration measurement and degradation, respectively. This study employs a recently developed unsaturated, one-dimensional horizontal flow method to determine the sorption of isoxaflutole's herbicidally active degradate, DKN, and dicamba. A 20-cm acrylic column was packed with soils varying in texture that had been uniformly treated with 14C-labeled chemical. The antecedent solution herbicide in equilibrium with sorbed phase herbicide was displaced by herbicide-free water, which was infiltrated into the column. Sorption coefficients, Kd, were obtained from a plot of herbicide concentration in soil vs water content in region where the antecedent solution was accumulated. DKN Kd values were ~5 times greater using the one-dimensional flow method as compared to the batch equilibration method in clay loam (0.71 vs 0.13 L kg-1), but similar for the two methods in sand (0.12 vs 0.08 L kg-1) soils. Dicamba Kd values were 1.5 times greater using the one-dimensional flow method as compared to the batch equilibration method in the clay loam soil (0.42 vs 0.33 L kg-1, however, the Kd values were the same for the two methods in the sand (~0.07 L kg-1). This demonstrates that to determine sorption coefficients for labile, hydrophilic pesticides an unsaturated flow, horizontal column may be a suitable alternative to the batch method. In fact, it may be better in cases where transport models have over predicted herbicide leaching when batch sorption coefficients have been used. Another advantage of this method is that selected soil hydraulic properties necessary for transport models are readily obtained.