Degradation of the Herbicide Metolachlor in Drummer Soil Under Different Redox Conditions.

Understanding the role of microorganisms and effect of soil environmental conditions on herbicide fate is critical for stewardship of herbicide use in cropping systems. As compared to the modernized perceptions of soil redox status, diminutive progress has been made in characterizing the impact of anaerobic micro sites and soil saturation on the biological fate of herbicides. Microcosm studies were undertaken to determine the dissipation of the herbicide [¹⁴C] metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(methoxy-1-methylethyl) acetamide] under aerobic and anaerobic environmental conditions in Drummer soil from Urbana, IL. Mineralized, aqueous, extractable and bound ¹⁴C-residues were quantified with LSC and characterized using HPLC and soil combustion.  Soil redox and Fe (II) status were monitored throughout the duration of incubation. The study revealed that the rate of aerobic and anaerobic degradation of metolachlor in drummer soil varied significantly during the initial period of incubation (until 8^th day) and showed no or little difference thereafter through the course of study. The disappearance of the herbicide from the anaerobic microcosm was evidently rapid till the detection of Fe (II) in the soil. Significant amount of unknown radioactivity was detected in both aerobic and anaerobic incubation which could be attributed to the presence of metabolites in the soil. Significant evolution of ¹⁴CO₂ was detected after the 8^th day in both aerobic and anaerobic soil incubation. Toward the end of experiment (128^th day), there appeared a trend in the direction of greater mineralization rate in anaerobic soil condition. Occurrence of ¹⁴CH₄ in the headspace at the end of the incubation indicated the presence of methanogenic conditions in anaerobic soil microcosm. About half of the applied radioactivity was eventually incorporated as soil-bound residue under both aerobic and anaerobic soil conditions.