FATE OF CHLORSULFURON IN THE ENVIRONMENT - 1 - LABORATORY EVALUATIONS

The behaviour and fate of chlorsulfuron in aqueous and soil systems we re examined in laboratory studies. Aqueous hydrolysis was PH-dependent and followed pseudo-first-order degradation kinetics at 25 degrees C, with faster hydrolysis occurring at pH 5 (half-life 24 days) than at either pH 7 or 9 (half-lives >365 days). Degradation occurred primaril y by cleavage of the sulfonylurea bridge to form the major metabolites chlorobenzenesulfonamide (2-chlorobenzenesulfonamide) and triazine am ine (4-methoxy-6-methyl-1,3,5-triazin-2-amine). This route is a major degradation pathway in water and soil systems. Aqueous photolysis (cor rected for hydrolysis) proceeded much more slowly (half-life 198 days) than aqueous hydrolysis and is not expected to contribute significant ly to overall degradation. Hydrolysis in soil thin-layer plates expose d to light (half-life 80 days), however, progressed at a much faster r ate than in dark controls (half life 130 days), which suggests that a mechanism other than direct photolysis may have been operative. An aer obic soil metabolism study (25 degrees C) in a Keyport silt loam soil (pH 6.4, 2.8% OM) showed that degradation was rapid (half-life 20 days ). Dissipation in an anaerobic sediment/water system (initial pH of wa ter phase 6.7, anal pH 7.4) progressed much more slowly(half-life >365 days) than in aerobic soil systems. Major degradation products in aer obic soil included the chlorobenzenesulfonamide and triazine amine as in the aqueous hydrolysis study. Neither of these degradation products exhibited phytotoxicity to a variety of crop and weed species in a gl asshouse experiment, and both exhibited an acute toxicological prone s imilar to that of chlorsulfuron in a battery of standard tests. Demeth ylation of the 4-methoxy group on the triazine moiety and subsequent c leavage of the triazine ring is another pathway bound in both aqueous solution and soils, though different bonds on the triazine amine appea r to be cleaved in the two systems. Hydroxylation of the benzenesulfon amide moiety is a minor degradation pathway found in soils. Two soils amended with 0.1 and 1.0 mg kg(-1) chlorsulfuron showed slight stimula tion of nitrification. The 1.0 mg kg(-1) concentration of chlorsulfuro n resulted in minor stimulation and inhibition of C-14-cellulose and C -14-protein degradation, respectively, in the same soils. Batch equili brium adsorption studies conducted on four soils showed that adsorptio n was low in this system (K-oc 13-54). Soil thin-layer chromatography of chlorsulfuron (R-f = 0.55-0.86) and its major degradation products demonstrated that the chlorobenzenesulfonamide (R-f = 0.34-0.68) had s lightly less mobility and that the triazine amine (R-f = 0.035-0.40) w as much less mobile than chlorsulfuron. In an aged column leaching stu dy, subsamples of a Fallsington sandy loam (pH(water) 5.6, OM 1.4%) or a Flanagan silt loam (pH(water) 6.4, OM 4.0%) were treated with chlor sulfuron, aged moist for 30 days in a glasshouse and then placed upon a prewet column of the same soil type prior to initiation of leaching. This treatment resulted in the retention of much more total radioacti vity (including degradation products) than by a prewet column, where i nitiation of leaching began immediately after chlorsulfuron applicatio n, without aging (primarily chlorsulfuron parent). (C) 1998 SCI.