Alachlor Biotransformation and Sorption in Soil from Two Soybean Tillage Systems

Few studies have investigated tillage effects on alachlor [2-chloro - N- (2,6-diethylphenyl) -N- (methoxymethyl)acetamide] degradation. Objectives here were to evaluate kinetics and mechanisms of alachlor degradation in surface (0−5 cm) Dundee silt loam (fine-silty, mixed, thermic, Aeric Ochraqualf) from plots managed for 7 years as conventional tillage (CT) or no-tillage (NT) soybeans ( Glycine max L.). In experiment I, incubations ranged from 0 to 54 days after 14 C-ring-labeled alachlor (4.44 μmol kg - 1 ) application. In experiment II, soil was incubated (10 g:3 mL of CaCl 2 ) with 14 C-labeled alachlor (14.8 or 111.2 μmol L - 1 ) from 1 to 25 days. Soils in experiment I were extracted with methanol and in experiment II were sequentially desorbed with 0.01 M CaCl 2 and methanol and then oxidized to measure unextractable 14 C. Mineralization of alachlor was more rapid in NT. Methanol-extractable and CaCl 2 -desorbable alachlor declined during incubation for both tillage treatments, with a corresponding increase in methanol-unextractable 14 C. Tillage did not influence the initial disappearance of alachlor (half-life ≈ 6.5 days) but did affect metabolite transformations. Alachlor sorption was more rapid in NT, accompanied by less CaCl 2 -desorbable and more methanol-extractable alachlor and metabolites. Unextractable 14 C was also greater in the NT soil. Acidic metabolites accumulated in both tillage soils.