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METRIBUZIN MOBILITY AND DEGRADATION IN UNDISTURBED SOIL COLUMNS

Metribuzin [4-amino-6-(1–1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] is a widely used soil-applied herbicide, and its dissipation was assessed using undisturbed soil columns to simulate field conditions. Metribuzin (technical grade and 14C-labeled dissolved in 0.1 M CaCl2) was applied to the soil (Dundee loam) surface at a concentration equivalent to 0.68 kg metribuzin ha−1. Water was added to the surface at 1, 8, 15, 22, 29, and 36 days after treatment. Chloride (Cl−) and 14C were measured in the effluent. Twenty-four hours after each leaching, three cores were randomly selected, sectioned at 0–3.3-, 3.3–6.6-, and 6.6–10-cm depth increments, and analyzed. Metribuzin mineralization was monitored weekly by trapping 14CO2-C in 0.5 M NaOH. The pattern of Cl− appearance in the effluent indicated possible preferential flow. Metribuzin degradation in the 10-cm profile followed first-order kinetics, with parent metribuzin being the dominant extractable species until the last 14 days when a composite of unidentified polar metabolites was prevalent. Metribuzin was the primary single component measured in the effluent until 37 days after application. However, from 9 days after herbicide application through the remainder of the experiment, the fraction of total applied 14C present as metabolites (summation of known metabolites diketometribuzin [DK], deaminated metribuzin [DA], and deaminated diketometribuzin [DADR] and unidentified polar metabolites) exceeded metribuzin in the effluent. As an average over time, the 3.3–6.6-cm section contained the most methanol unextractable 14C