Fate of chlorsulfuron in the environment. 2. Field evaluations

The fate and mobility of chlorsulfuron was determined in several field studies with 14C-labeled chlorsulfuron. A study comparing fall with spring applications (∽100 g AI ha-1) to in-situ soil columns (35 cm depth) in neutral to alkaline soils (pH 6·9–8·2, OM 1·0–5·3) located in CO, ID and ND demonstrated that fall treatments did not persist longer into the following year than spring treatments. Mobility into the soil profile appeared to be initially faster following fall applications at the ID and ND sites, but differences between application seasons appeared to moderate with time. A field-soil metabolism study conducted at Madera, CA on a sandy loam soil (pH 6·3–6·9 and 0·3–0·4% OM with depth) with chlorsulfuron (∽158 g AI ha-1) demonstrated rapid dissipation of chlorsulfuron (pseudo-first-order half-life 18 days). No intact chlorsulfuron was found after the 120-day sampling. Major metabolites observed in this study were chlorobenzenesulfonamide (2-chlorobenzenesulfonamide) and triazine amine (4-methoxy-6-methyl-1,3,5-triazin-2-amine), products of bridge cleavage, and O-desmethylchlorsulfuron (1-(2-chlorophenylsulfonyl)-3-(4-hydroxy-6-methyl-1,3,5-triazin-2-yl)urea). No intact chlorsulfuron was detected below the 0–15 cm layer at any sampling (maximum depth 60–90 cm), but chlorobenzenesulfonamide and ring-opened carbamoyl guanidine (1-(2-chlorophenylsulfonyl)-3-(ureido-imino)urea) were found at the 15–30 cm depth. In a similar study conducted on a silt loam soil in Moscow, ID (pH 6·1–6·9 and 2·2–1·0% OM with depth), overall dissipation was much slower than at Madera, CA due to the cooler climate (average soil temperature 8·6° versus 20·0°). The initial rate of chlorsulfuron dissipation was similar (pseudo-first-order half-life 18 days), but dissipation exhibited a distinctly slower second stage (half-life 109 days) not exhibited at Madera, CA. By the 370-day sampling no intact chlorsulfuron was detected. The chlorobenzenesulfonamide and triazine amine were the major metabolites found in this study, accounting for approximately 38 and 30%, respectively, of the initial chlorsulfuron at the last sampling (571 days). Other metabolites were found in lesser amounts, including O-desmethylchlorsulfuron, ring-opened carbamoyl guanidine, hydroxy triazine amine (4-amino-6-methyl-1,3,5-triazine-2-ol), triazine urea ((4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea), an undifferentiated bound fraction and an unidentified group of polar components. The presence of triazine urea indicates that soil-surface photolysis (or indirect photolysis) may have been operative. In the study in Moscow, ID, no intact chlorsulfuron was found below the 0–15 cm layer at any sampling (maximum depth 75 cm). Movement of total radioactive components was restricted to a maximum depth of 60 cm at Madera, CA and 50 cm at Moscow, ID. The overall water balance over the duration of both studies was negative, helping to explain the observed lack of leaching. The PRZM3 model was used to predict the distribution profile of chlorsulfuron at the Moscow, ID site for which the base case overpredicted leaching (down to 20–30 cm) and predicted the depletion of the surface layer, which did not occur. The prediction was improved by arbitrarily doubling the Koc value and using a slightly higher than measured soil bulk density. © 1998 SCI