Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2002
Publication Date: 12/20/2002
Citation: RICE, P.J., ANDERSON, T.A., COATS, J.R. DEGRADATION AND PERSISTENCE OF METOLACHLOR IN SOIL: EFFECTS OF CONCENTRATION, SOIL MOISTURE, SOIL DEPTH, AND STERILIZATION. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. 21(12):2640-2648. Interpretive Summary: The persistence and availability of pesticide residues in soil will impact their movement from an area of application to surrounding surface water or ground water. The present study was conducted to determine the influence of soil moisture, pesticide concentration and soil depth on the degradation of metolachlor and formation of soil-bound residues. Increased soil moisture increased degradation, which resulted in greater non-extractable residues. Degradation was greater in the surface soil than in the subsurface soil regardless of the soil moisture or the initial metolachlor concentration. Understanding factors that influence the persistence of metolachlor in soil will allow for more accurate prediction of the environmental fate of this herbicide. These results will be of benefit/interest to farmers, extension agents and scientist who will use it to facilitate the development of management factors that will decrease the potential mobility of herbicides toward water resources.
Technical Abstract: The aim of our investigation was to evaluate the influence of soil depth, soil moisture and concentration on the persistence and degradation of metolachlor in soil. Greater percentages of metolachlor persisted in subsurface soils than surface soil regardless of the soil moisture or initial herbicide concentration. Larger quantities of bound residues and extractable degradation products were found in the surface soils as a result of the increased soil sorption and biodegradation of metolachlor associated with the surface soil which had more organic matter. Saturated soil favored the dissipation of metolachlor and the formation of soil-bound residues. Significantly greater quantities of a dechlorinated metabolite were measured in the saturated surface soil compared to the unsaturated soil. Mineralization of metolachlor to CO2 and volatilization of metolachor or metolachlor degradates was minimal in surface and subsurface soils at both soil moistures and herbicide concentrations. Increased metolachlor concentrations did not inhibit microbial activity, however, the greater rate of application did result in the reduced percentage of applied [14C]metolachlor that was bound to surface or subsurface soil. A significant reduction in the quantity of extractable metolachlor degradates and unextractable soil-bound residues in sterile soil revealed the significance of biodegradation to the dissipation of metolachlor in soil.