Mineralization of 2,4-D and atrazine in the unsaturated zone of a sandy loam soil

Abstract

Pesticide transformation in the unsaturated zone may greatly reduce the amount of pesticide leaching to groundwater. Understanding and quantifying the fate of pesticides in the unsaturated zone will greatly improve the accuracy of pesticide leaching models and enable more effective groundwater policies to be devised. We studied the effects of temperature, moisture and initial pesticide concentration on mineralization of 2,4-D and atrazine at different soil depths down to 1.5 m. Mineralization of 2,4-D was followed for 2.5 months and atrazine mineralization for 1 month. Uniformly ring-labelled 2,4-D and atrazine were applied to soil to give initial pesticide concentrations between 7.5 and 60 μg kg −1 for 2,4-D and between 100 and 600 μg kg −1 for atrazine. Moisture contents varied between 15 and 40% of moisture holding capacity and temperatures varied between 4 and 20°C. Mineralization of atrazine was always low (<2%) and decreased with depth, lower temperatures and lower moisture contents. Microbial biomass (r 2 > 0.66) and soil organic carbon (r 2 > 0.76) were positively correlated with atrazine mineralization rates. Between 10 and 80% of applied 2,4-D was completely mineralized depending on the test conditions and soil depth. 2,4-D mineralization rates were generally highest at depths between 1 and 1.5 m and on average 4 times higher than in the surface soils. Variations in 2,4-D mineralization rates with depth could not be correlated to changes in soil organic C, soil biomass, or chemical availability, and are probably the result of complex interactions between microbial activity, community structure, nutrient status and soil chemical and physical properties. As with atrazine, 2,4-D mineralization increased with increasing temperature and moisture contents. Addition of glucose reduced the mineralization of 2,4-D but enhanced atrazine mineralization, likely indicating differences in metabolic pathways involved.