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United States Department of Agriculture

Agricultural Research Service

Title: Effect of Tillage on Alachlor Sorption and Biotransformation in Soil

Authors
item Locke, Martin
item Zablotowicz, Robert
item Gaston, Lewis

Submitted to: International Weed Control Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 28, 1996
Publication Date: N/A

Interpretive Summary: Effects of tillage on alachlor sorption and biotransformation in soil were evaluated in a series of laboratory studies using Dundee silt loam (fine-silty, mixed, thermic, Aeric Ochraqualf) soil composited from the surface (0-5 cm) of long-term conventional tillage (CT) or no-tillage (NT) soybean (Glycine max. L.) plots. Sorption and desorption K values were greater for NT for each of the sorption shaking times. Kinetics were described using a three-site model and exhibited two phases with rapid initial sorption up to 3 h followed by slower sorption. Labile and restricted fractions increased with time, especially for NT. Faster and higher total alachlor sorption in NT was attributed to higher soil organic residues. Studies monitoring biotransformations of alachlor included incubation periods ranging from 0 to 54 days. Soils were extracted with methanol or 0.01 M CaCl2, and oxidized. Herbicide mineralization was more rapid and methanol-unextractable 14**C was higher in Nt. Tillage did not influence initial alachlor disappearance, but did affect metabolite transformations. Acidic metabolites (e.g., oxanilic and sulfonic acid) persisted longer in the CT soil. Our studies have demonstrated that several genera of Gram-negative bacteria have the potential to dechlorinate alachlor via glutathione conjugation to acidic metabolites.

Technical Abstract: Objectives were to review a series of studies evaluating the kinetics and mechanisms of alachlor sorption and biotransformation in soil from two long-term no-history soybean tillage systems. Batch sorption methods were used where soil samples were shaken for selected times, sequentially desorbed, extracted with methanol, and oxidized. Alachlor biotransformation nstudies included incubation periods from 0 to 54 days. At selected times, soils were extracted and oxidized. Under no-tillage (NT), microbial populations were significantly greater than for conventional tillage (CT). Several genera of Gram-negative bacteria were found to have the potential to dechlorinate alachlor via glutathione (GSH) conjugation to acidic metabolites. Sorption and desorption K values were greater for NT for each of the sorption shaking times. Sorption kinetics were described using a three-site model and exhibited two phases with rapid initial sorption up to 3 h followed by slower sorption. Labile and restricted fractions increased with time, especially for NT. Faster and higher total alachlor sorption in NT was attributed to higher soil organic residues. Herbicide mineralization was more rapid and methanol-unextractable 14**C was higher in NT. Tillage did not influence initial alachlor disappearance, but did affect metabolite transformations. Acidic metabolites (eg. oxanilic and sulfonic acid) persisted longer in the CT soil.

Last Modified: 9/1/2014
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