Enantioselective Degradation of Metalaxyl in Soils:  Chiral Preference Changes with Soil pH

Chiral pesticides are often degraded enantio-/stereoselectively in soils. Degradation is typically studied with one or a small number of soils so that it is not possible to extrapolate the findings on chiral preference to other soils. For this study, the fungicide metalaxyl was chosen as a “chiral probe” to investigate its enantioselective degradation in 20 different soils, selected primarily to cover a wide range of soil properties (e.g., acidic/alkaline, aerobic/anaerobic) rather than to consider soils of agricultural importance. Racemic metalaxyl was incubated in these soils under laboratory conditions, and the degradation of the enantiomers as well as the enantioselective formation/degradation of the primary major metabolite, metalaxyl acid, was followed over time, using enantioselective GC-MS after ethylation with diazoethane. In aerobic soils with pH > 5, the fungicidally active R-enantiomer was degraded faster than the S-enantiomer (k_R > k_S), leading to residues with a composition [S] > [R]. However, in aerobic soils with pH 4−5, both enantiomers were degraded at similar rates (k_R ≈ k_S), and in aerobic soils with pH < 4 and in most anaerobic soils, the enantioselectivity was reversed (k_R < k_S). These considerable soil-to-soil variations were observed with soils from locations close to each other, in one case even within a single soil profile. Liming and acidification of a “nonenantioselective” soil prior to incubation resulted in enantioselective degradation with k_R > k_S and k_R < k_S, respectively. While the enantioselectivity (expressed as ES = (k_R − k_S)/(k_R + k_S)) of metalaxyl degradation in aerobic soils apparently correlated with soil pH, no such correlation was found for metalaxyl acid. Reevaluation of published kinetic data for the herbicides dichlorprop and mecoprop indicated similar correlations between soil pH and ES as for metalaxyl.