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Sorption-Desorption Kinetics of Alachlor in Surface Soil from Two Soybean Tillage Systems

Southern Weed Science Lab., USDA-ARS, P.O. Box 350, Stoneville, MS 38776.

^* Corresponding author.

ABSTRACT

Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl)-acetamide] sorption patterns were evaluated in surface (0–5 cm) Dundee silt loam (fine-silty, mixed, thermic Aeric Ochraqualf) soil collected from long-term conventional tillage (CT) and no-tillage (NT) soybean [Glycine max (L.) Merr.] plots. Three concentrations of [¹⁴C] alachlor (0.58, 2.44, and 18.1 µmol L^–1) in 0.01 M CaCl² were added to soil (3 g/18 mL) and samples were shaken for time periods ranging from 1 min to 96 h. The difference between initial and final alachlor concentration was attributed to sorption. Desorption (0.01 M CaCl₂) was measured for samples initially sorbed for 1, 24, or 96 h. Desorbed soils were extracted with methanol and then combusted. Freundlich parameters (K_f [1/n]) were calculated for alachlor sorption and desorption using nonlinear regression. Sorption K_f values for soil from both tillage systems increased as sorption shaking time increased. Desorption was hysteretic, and length of sorption shaking increased non-singularity. Sorption K_f values measured for NT were greater than CT for each of the 1, 24, and 96 h sorption shaking times. Similarly, desorption K_f values for NT were greater than for CT. Kinetics were evaluated using a three-site model. Sorption patterns indicated rapid initial sorption up to approximately 3 h followed by slower sorption. No change in solution alachlor was observed after 48 h for either tillage treatment. Labile and restricted fractions increased with time, especially for NT. The increased rate and overall quantity of alachlor sorption in NT was attributed to higher soil organic residues.

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