Effect of soil moisture content on Metribuzin degradation in a sandy soil

The effects of different soil moisture levels on the degradation of herbicide metribuzin (4-amino-6-[ 1,1dimethylethyl]- 3-[methylthio]-1 ,2,4-triazin-5[4H]-one) were investigated under laboratory conditions. Soil typical of southern Quebec, St. Amable sand (Humic Haplorthod), was collected from a field with no history ofpesticide application for the previous five years and used in the laboratory experiment. Five volumetric moisture content levels, in triplicate, of 10, 15, 25, 35, and 45% and one air dried soil were used to represent different moisture regimes. Soil samples were collected from all treatments 3, 7, 14, 30, 60, and 90 days after metribuzin application and were analyzed with a gas chromatograph for metribuzin residues. Results indicate that the degradation rate of metribuzin was significantly higher (P<0.05) at 25 and 35% soil moisture contents than that at all other moisture levels. The metribuzin decay kinetics were expressed with a first order reaction equation with R2 values ranging from 0.78 to 0.94 for different moisture levels. The half-life of metribuzin varied from 3 to 17 days at 21 ± 2 °C for different moisture treatments. Shortest halflives were obtained for soil moisture levels of 25 and 35% and longer half-lives resulted for soil at lower moisture levels and at saturation. Therefore, based on the results of this experiment, it can be concluded that watertable management systems for humid regions which maintain higher soil moisture content in the crop root zone may also be effective in enhancing the quick degradation of metribuzin in sandy soils. Such systems can reduce the risk of metribuzin leaching towards groundwater.