Degradation of the herbicide, acetochlor in surface and sub-surface soils under field and laboratory conditions

Rapid and comparable rates of acetochlor degradation were found in surface and subsoils incubated under both field and laboratory conditions. For the field-incubation study, undisturbed soil cores were sampled vertically from surface (0-30 cm) and two sub-surface (30-76 cm and 260-305 cm) depths. Soil cores were treated in the field with C-14-labelled acetochlor and reburied in the field at a depth similar to their depth of abstraction. Laboratory-incubated soils from the same site were incubated under two separate sets of conditions: 20 degrees C/moisture content pF2, and temperature/moisture conditions similar to field incubation conditions. In both studies, acetochlor was applied to surface soil at a rate of 2.9 kg ha(-1), and to the subsoils at approximately 5% of this rate. Rates of acetochlor degradation were faster in subsoils (7.5 to 13.8 days) than surface soil (18.3 days) in field-incubated soils. Furthermore, field degradation rates were comparable to laboratory-generated data (subsoils 2-32 days; surface soil 8-15 days). This confirms the validity of performing laboratory-based degradation studies to determine pesticide DT50 values in subsoils.