Degradation and Persistence of Metolachlor in Soil: Effects of Concentration, Soil Moisture, Soil Depth, and Sterilization

Abstract

The present study evaluated 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 in 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 metolachlor 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.

Sorption and dissipation of aged metolachlor residues in eroded and rehabilitated soils

“…Unextracted residues at 8 weeks after conventional extraction accounted for about 45% of the 14 C added to the soil, significantly greater than for ASE (∼32%) because of the lower efficiency of manual extraction (Table 2). The formation of soil‐bound residues has been shown by others to be important in metolachlor dissipation 30, 43…”

Mineralisation of atrazine, metolachlor and their respective metabolites in vegetated filter strip and cultivated soil

“…With the exception of Kotoula‐Syka et al ,46 who reported ⩽8% cumulative mineralisation of 14 C‐ ring ‐labelled metolachlor after 52 days of incubation, our data for the cumulative mineralisation of metolachlor in cultivated soil are greater than published results (Table 4). Reported values for the mineralisation of 14 C‐ ring ‐labelled metolachlor in cultivated soil are ⩽4% after 46 days of incubation, ⩽1.5% after 120 days of incubation and ⩽1.6% after 49 days of incubation 21, 47, 48. Similarly, our values for the cumulative mineralisation of metolachlor in VFS soil are greater than those of Staddon et al ,21 who reported ⩽4.0% cumulative mineralisation of 14 C‐ ring ‐labelled metolachlor after 46 days of incubation.…”

Adsorption, desorption and dissipation of metolachlor in surface and subsurface soils

“…Microbial degradation is the primary means of metolachlor dissipation in soil 21. Factors that influence microbial activity, especially soil temperature and moisture content, in turn influence the persistence of metolachlor 22, 23. To date, most of the studies on metolachlor degradation have focused on the surface soil and on dissipation and leaching into the unsaturated zone, and only a few investigations have considered degradation in the subsoil 22, 24.…”

“…Factors that influence microbial activity, especially soil temperature and moisture content, in turn influence the persistence of metolachlor 22, 23. To date, most of the studies on metolachlor degradation have focused on the surface soil and on dissipation and leaching into the unsaturated zone, and only a few investigations have considered degradation in the subsoil 22, 24. However, as even small amounts of pesticides can have adverse effects on groundwater quality,25 information on microbial degradation in the subsoil is important for an understanding of the overall fate of metolachlor.…”