Soil Cover and Tillage Influenced Metolachlor Mobility and Dissipation in Field Lysimeters

Leaching studies using soil column field lysimeters were conducted in 1991 and 1992 to determine the influence of tillage [conventional (CT) and reduced (RT)], soil cover (fallow, soybean, and bermudagrass sod), and water input level on 14C metolachlor mobility. Runoff and leachate collectors were installed. Leachate was collected weekly and analyzed for herbicide content. At 128 d after treatment (DAT), the lysimeters were removed, and the soil was analyzed for herbicide concentration as a function of depth. In 1991, 10% greater amounts of 14C metolachlor volatilized from CT than from RT (32 vs. 22%). Amounts of metolachlor recovered in surface cover were 2.3% less (2.1 vs. 4.4%), amounts in subsoil were 8% less (47 vs. 55%), and amounts recovered in leachate were 0.7% less (0.7 vs. 1.4%) under CT than RT. In 1992, measured parameters of 14C distribution in soil for tillage treatments were similar, except 14C recovered in leachate was 3% greater under RT (7 vs. 4%). Amounts of 14C metolachlor retained by soil cover were greatest in bermudagrass sod (16%), less in soybean (4%), and least in fallow (1%) and were similar for each treatment both years. Carbon‐14 metolachlor recovered in leachate of all treatments was approximately five times greater in 1992 (5%) than in 1991 (1%) due to the 7% greater water input in 1992, much of which occurred shortly after Day 0. Leachate volume collected in 1992 (8 L) was also twice that of 1991 (4 L). Tillage and soil cover interaction effects occurred both years.