Fate of chlorsulfuron in the environment. 1. Laboratory evaluations

Abstract

The behaviour and fate of chlorsulfuron in aqueous and soil systems were examined in laboratory studies. Aqueous hydrolysis was pH-dependent and followed pseudo-first-order degradation kinetics at 25°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 primarily by cleavage of the sulfonylurea bridge to form the major metabolites chlorobenzenesulfonamide (2-chlorobenzenesulfonamide) and triazine amine (4-methoxy-6-methyl-1,3,5-triazin-2-amine). This route is a major degradation pathway in water and soil systems. Aqueous photolysis (corrected for hydrolysis) proceeded much more slowly (half-life 198 days) than aqueous hydrolysis and is not expected to contribute significantly to overall degradation. Hydrolysis in soil thin-layer plates exposed to light (half-life 80 days), however, progressed at a much faster rate than in dark controls (half life 130 days), which suggests that a mechanism other than direct photolysis may have been operative. An aerobic soil metabolism study (25°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 water phase 6·7, final pH 7·4) progressed much more slowly (half-life >365 days) than in aerobic soil systems. Major degradation products in aerobic 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 glasshouse experiment, and both exhibited an acute toxicological profile similar to that of chlorsulfuron in a battery of standard tests. Demethylation of the 4-methoxy group on the triazine moiety and subsequent cleavage of the triazine ring is another pathway found in both aqueous solution and soils, though different bonds on the triazine amine appear to be cleaved in the two systems. Hydroxylation of the benzenesulfonamide moiety is a minor degradation pathway found in soils. Two soils amended with 0·1 and 1·0 mg kg-1 chlorsulfuron showed slight stimulation of nitrification. The 1·0 mg kg-1 concentration of chlorsulfuron resulted in minor stimulation and inhibition of 14C-cellulose and 14C-protein degradation, respectively, in the same soils. Batch equilibrium adsorption studies conducted on four soils showed that adsorption was low in this system (Koc 13–54). Soil thin-layer chromatography of chlorsulfuron (Rf=0·55–0·86) and its major degradation products demonstrated that the chlorobenzenesulfonamide (Rf=0·34–0·68) had slightly less mobility and that the triazine amine (Rf=0·035–0·40) was much less mobile than chlorsulfuron. In an aged column leaching study, subsamples of a Fallsington sandy loam (pHwater 5·6, OM 1·4%) or a Flanagan silt loam (pHwater 6·4, OM 4·0%) were treated with chlorsulfuron, 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 radioactivity (including degradation products) than by a prewet column, where initiation of leaching began immediately after chlorsulfuron application, without aging (primarily chlorsulfuron parent). © 1998 SCI