Aerobic soil metabolism of metsulfuron-methyl

A laboratory study was conducted to determine the degradation rates and identify major metabolites of the herbicide metsulfuron- methyl in sterile and non- sterile aerobic soils in the dark at 20 degree C. Both [phenyl-U- super(14)C]- and [triazine-2- super(14)C] metsulfuron-methyl were used. The soil was treated with [ super(14)C] metsulfuron- methyl (0.1 mg kg super(-1)) and incubated in flow-through systems for one year. The degradation rate constants, DT sub(50), and DT sub(90) were obtained based on the first-order and biphasic models. The DT sub(50) (time required for 50% of applied chemical to degrade) for metsulfuron-methyl, estimated using a biphasic model, was approximately 10 days (9-11 days, 95% confidence limits) in the non-sterile soil and 20 days (12-32 days, 95% confidence limits) in the sterile soil. One-year cumulative carbon dioxide accounted for approximately 48% and 23% of the applied radioactivity in the [phenyl-U- super(14)C] and [triazine-2- super(14)C] metsulfuron-methyl systems, respectively. Seven metabolites were identified by HPLC or LC/MS with synthetic standards. The degradation pathways included O-demethylation, cleavage of the sulfonylurea bridge, and triazine ring opening. The triazine ring-opened products were methyl 2-[[[(acetylamino) carbohyl]amino]carbonyl] amino]carbonyl] -amino]sulfonyl] benzoate in the sterile soil and methyl 2-[[[amino[(amino-carbonyl) imino]methyl] amino]carbonyl] amino]sulfonyl]benzoate in the non-sterile soil, indicating that different pathways were operable.