Environ. Sci. Technol. 2003, 37, 707-712
Degradation of Racemic and Enantiopure Metalaxyl in Tropical and Temperate Soils ADOLPHE MONKIEDJE,† MICHAEL SPITELLER,* AND KAI BESTER Institute of Environmental Research, University of Dortmund, Otto-Hahn-Str. 6, D-44227 Dortmund, Germany
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 O1 column. The degradation followed first-order kinetics (R2 g 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.
Introduction The phenylamide fungicide, metalaxyl, is a chiral compound (structural formula in Figure 1), which is marketed in its racemic as well as in its enantiopure R form (trade names, e.g., mefenoxam, ridomil, etc.). Metalaxyl has two enantiomers which are expected to be formed in a 1:1 ratio if synthesized from racemic materials (1); however, the enantiomeric distribution should be checked, as some assumed racemates do not give 1:1 peak ratios. Metalaxyl was introduced in 1977 and is used as a seed treatment, a banded or broadcast soil application, and a foliar spray. Sometimes, it is used in combination with protectant type fungicides such as copper, to control plant diseases caused by oomycetes. It is a systemic, apoplasticallytransported fungicide, highly active against fungi of the order Peronosporales. Because of its broad-spectrum activity, this compound is registered for use on a wide range of crops and in many countries and in temperate and tropical regions. The product “mefenoxam” consists of R-metalaxyl, which is * Corresponding author phone: + 49-231-755-4080; fax: + 49231-755-4085; e-mail:
[email protected]. † Present address: Laboratory of General Biology, Department of Animal Biology and Physiology, Faculty of Science, University of Yaounde I, P.O. Box 812, Cameroon. E-mail:
[email protected]. 10.1021/es020123e CCC: $25.00 Published on Web 01/08/2003
2003 American Chemical Society
FIGURE 1. Structures of the two enantiomers of metalaxyl. the only fungicidal active enantiomer. It, thus, provides the same level of efficiency as metalaxyl but at half the application rate. The recent introduction of mefenoxam onto the market was aimed at reducing the risk posed by metalaxyl to the environment (2). Metalaxyl has been shown to affect soil biology adversely. Recommended commercial use rates of metalaxyl have been shown to affect the growth of soil microorganisms adversely (3). Applications of this pesticide on vineyard soils over 3 years markedly decreased microbial numbers and activity and increased the number of microorganisms involved in the mineralization of organic matter (4). The systemic application of metalaxyl induced a brief outbreak and subsequent suppression of soil fungi and actinomycetes. Moreover, metalaxyl has been found in food (5-7). The importance of these environmental side effects of metalaxyl depend largely on its persistence in the soils, which is influenced by the activity of soil microorganisms (8). Many reports have documented the microbially mediated degradation of metalaxyl in soils (9-11) and the faster degradation of the R-metalaxyl in temperate soil (1, 12-14). These reports have not addressed degradation behavior of the enantiomers applied in various formulations of metalaxyl in soils of different climatic environments. Information about pesticide dissipation with time is essential in assessing environmental risks (15). The replacement of racemic metalaxyl by, e.g., mefenoxam (16), means that knowledge of and data on the persistence of this enantiomer are essential for use, management and its registration, especially in the tropical regions of Africa, e.g., Cameroon, where metalaxyl is heavily used in cocoa farming (3). Additionally, knowledge on the degradation mechanism is a prerequisite for the registration in new fields of application. The objective of this work was to study the enantioselective degradation and persistence of the racemic and enantiopure forms of metalaxyl in tropical and temperate soils using reverse phase and chiral high-performance liquid chromatography (HPLC). Additionally, it was the aim of the study to gain information on the effect of the formulation on degradation.
Experimental Section Analytical standards (purity, >99%) of unformulated racemic metalaxyl and racemic metalaxyl acid metabolite (MX-acid) were obtained from Riedel-de Haen, Germany and Novartis AG (Basle, Switzerland), respectively. Formulated racemic metalaxyl and formulated enantiopure R-metalaxyl were obtained as emulsifiable concentrate (EC) formulations containing 24% and 48% of metalaxyl and R-metalaxyl respectively from Novartis Agro GmbH, Frankfurt, Germany. VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Physicochemical Properties of the Soils Investigated texture analysis (USDA)
sand clay loam
sandy loam
clay (
