Effect of Soil Conditions on the Degradation of Cloransulam-Methyl

Herbicide efficacy and environmental fate are often controlled by soil conditions. Aerobic soil laboratory studies were undertaken to determine the degradation of the herbicide cloransulam-methyl [N-(2-carbomethoxy-6-chloro-phenyl)-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°C), moisture (20, 40, and 60% water filled pore space), and soil type. The soils under study were a Drummer silty clay loam (fine-silty, mixed, super-active, mesic Typic Endoaquolls) and a Cisne silt loam (fine, smectitic, mesic Vertic Albaqualfs). Variability in molecular degradation was investigated using two radiolabeled forms ([Phenyl-UL-¹⁴C] and [Pyrimidine-7,9-¹⁴C]). Dissipation of parent compound in soil solution and sorbed phases, formation of radiolabeled metabolites, ¹⁴C mineralization, total 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 ¹⁴C mineralization, however, was dependent on position of radiolabel, suggesting that distinct groups of microorganisms degrade different parts of the molecule at higher temperatures. Only ¹⁴C mineralization was influenced by moisture, with response depending on soil type. Increasing moisture resulted in more ¹⁴C mineralization in Drummer, but not Cisne soil, which was attributed to increased microbial access to pesticide at greater moisture contents in Drummer soil. Reduced availability, suggested by greater sorption in Drummer soil, may explain persistence of parent in this soil. Bound residues were more extensive and exhibited greater dependence on biological activity in Cisne soil, owing to enhanced dissipation of parent compound in this soil.