Degradation of racemic and enantiopure metalaxyl in tropical and temperate soils

The degradation of the racemic mixture and the enantiomers of metalaxyl in typical soils from Germany and Cameroon has been studied. Formulated and unformulated R-metalaxyl were studied as well as racemic (rac) metalaxyl in controlled incubation experiments. The kinetics of the degradation or transformation were determined by means of reversed phase HPLC, while the enantiomeric ratios were measured by HPLC with a chiral Whelk 01 column. The degradation followed first-order kinetics (R-2 greater than or equal to 0.96). Higher metalaxyl acid metabolite concentrations were found in German soil than in Cameroonian soil. The enantiomers of the fungicide each had different degradation rates in both soils, with half-lives ranging from 17 to 38 days. All forms of metalaxyl had lower degradation rates in the Cameroonian soil than in the German soil. The degradation of the R-enantiomer was much faster than the S-enantiomer in the German soil and slower than the S-enantiomer in the Cameroonian soil suggesting that different microbial populations, which may be using different enzymes, have different degradation preferences. The results for the major differences in the degradation of the enantiomers may have some implication for the frequency of use as well as the environmental assessment for chiral pesticides.