FATE OF CHLORSULFURON IN THE ENVIRONMENT - 2 - FIELD EVALUATIONS

The fate and mobility of chlorsulfuron was determined in several field studies with C-14-labeled chlorsulfuron. A study comparing fall with spring applications (similar to 100 g Al ha(-1)) to in-situ soil colum ns (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 pe rsist longer into the following year than spring treatments. Mobility into the soil profile appeared to be initially faster following fall a pplications at the ID and ND sites, but differences between applicatio n seasons appeared to moderate with time. A held-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 (similar to 158 g AI ha(-1)) demons trated rapid dissipation of chlorsulfuron (pseudo-first-order half-lif e 18 days). No intact chlorsulfuron was found after the 120-day sampli ng. Major metabolites observed in this study were chlorobenzenesulfona mide (2-chlorobenzenesulfonamide) and triazine amine (4-methoxy-6-meth yl-1,3,5-triazin-2-amine), products of bridge cleavage, and O-desmethy lchlorsulfuron l)-3-(4-hydroxy-6-methyl-1,3,5-triazin-2-yl)urea). No i ntact chlorsulfuron was detected below the 0-15 cm layer at any sampli ng (maximum depth 60-90 cm), but chlorobenzenesulfonamide and ring-ope ned carbamoyl guanidine (1-(2-chlorophenylsulfonyl)-3-(ureido-imino)ur ea) 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 co oler climate (average soil temperature 8.6 degrees versus 20.0 degrees ). The initial rate of chlorsulfuron dissipation was similar (pseudo-f irst-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 chlo robenzenesulfonamide and triazine amine were the major metabolites fou nd in this study, accounting for approximately 38 and 30%, respectivel y, of the initial chlorsulfuron at the last sampling (571 days). Other metabolites were found in lesser amounts, including O-desmethylchlors ulfuron, ring-opened carbamoyl guanidine, hydroxy triazine amine (4-am ino-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 u nidentified group of polar components. The presence of triazine urea i ndicates that soil-surface photolysis (or indirect photolysis) may hav e 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 maximu m depth of 60 cm at Madera, CA and 50 cm at Moscow, ID. The overall wa ter balance over the duration of both studies was negative, helping to explain the observed lack of leaching. The PRZM3 model was used to pr edict the distribution prone of chlorsulfuron at the Moscow, ID site f or which the base case overpredicted leaching (down to 20-30 cm) and p redicted the depletion of the surface layer, which did not occur. The prediction was improved by arbitrarily doubling the K-oc value and usi ng a slightly higher than measured soil bulk density. (C) 1998 SCI.