Effect of soil conditions on the degradation of cloransulam-methyl

Herbicide efficacy and environmental fate are often controlled by soil cond itions. Aerobic soil laboratory studies were undertaken to determine the de gradation of the herbicide cloransulam-methyl [N-(2-carbomethoxy-6-chloro-p henyl)-5-ethoxy-7-fluoro[1,2,4]triazolo[1,5-c]pyrimidine-2-sulfonamide] for a range of soil factors. Treatments included soil temperature (5, 15, 25, 40, and 50 degrees C), moisture (20, 40, and 60% water filled pore space), and soil type. The soils under study were a Drummer silty clay loam (fine-s ilty, mixed, superactive, mesic Typic Endoaquolls) and a Cisne silt loam (f ine, smectitic, mesic Vertic Albaqualfs). Variability in molecular degradat ion was investigated using two radiolabeled forms ([Phenyl-UL-C-14] and [Py rimidine-7,9-C-14]). Dissipation of parent compound in soil solution and so rbed phases, formation of radiolabeled metabolites, C-14 mineralization tot al microbial respiration, and bound residue formation were measured for up to 120 d. Dissipation of parent and formation of bound residues in Drummer soil increased with greater temperatures. The influence of temperature on C -14 mineralization, however, was dependent on position of radiolabel, sugge sting that distinct groups of microorganisms degrade different parts of the molecule at higher temperatures. Only C-14 mineralization was influenced b y moisture, with response depending on soil type. Increasing moisture resul ted in more C-14 mineralization in Drummer, but not Cisne soil, which was a ttributed to increased microbial access to pesticide at greater moisture co ntents in Drummer soil. Reduced availability, suggested by greater sorption in Drummer soil, may explain persistence of parent in this soil. Bound res idues were more extensive and exhibited greater dependence on biological ac tivity in Cisne soil, owing to enhanced dissipation of parent compound in t his soil.