Degradation of Metalaxyl under Tropical Conditions in a Field Trail

Aim

The phenylamid fungicide metalaxyl [N-(2, 6-dimethylphenyl)-N-(2'-methoxy acetyl)-DL-alanine, methyl ester] is a systemic fungicide that is widely used in agriculture and on horticultural crops for the control of root and stem diseases. Metalaxyl is a chiral compound on account of the presence of a stereogenic centre in the carboxy alkyl moiety. The use of mefenoxam is likely to increase in near future on account of its greater efficacy. We have investigated the behaviour of racemic metalaxyl and metalaxyl-M (Mefenoxam) in two different formulations in a tropical rainforest soil under field conditions, as well as to verify and monitor the possible changes in enantiomeric ratio (ER). In addition, we investigated the influence of metalaxyl and mefenoxam applied in various formulations on selected soil parameters.

Structure of S- and R-metalaxyl

Approach

A field experiment was conducted to fulfil the above objectives with a single application of metalaxyl- and mefenoxam-based EC (emulged concentrate) and WPC (powder, containing cupper ions) formulation at the recommended application rate under tropical rainforest field conditions. The kinetics of the degradation/dissipation of metalaxyl and its acid metabolite (metalaxylacid) were investigated using reverse phase HPLC-MS/MS, while the enantiomeric ratios were measured by HPLC/MS/MS using a chiral normal phase HPLC column. Metazachlor was used as internal standard. The identity of the compounds was confirmed by comparing retention times and multiple reaction monitoring (SRM) signals in tandem mass spectrometry (TSQ Quantum Ultra AM equipped with an APCI source). For (+) APCI, ionization conditions were optimized for every compound and two selected product ions were monitored for each precursor ion. The MS/MS parameters of every SRM chromatogram are listed below.

SRM chromatograms of metalaxyl, the acid metabolite and the internal standard (IS) Metazachlor of a soil extract 15 days after incubation

The ER of the compounds was determinded by using the relation of peak areas of the R and S isomers after chiral separation.

The microbial activity indices (enzymatic activities like acid/alkaline phosphatase and raw: β-glucosidase activities) were determined by established methods.

Soil sampling in Yaounde (Cameroon) Adolphe Monkiedje (right)

Results

The degradation of metalaxyl and metalaxyl-M in the soil from Cameroon was shown to be enantioselective, with the R-enantiomer being degraded more slowly than the S-enantiomer. Degradation followed approximate square root first-order kinetics (R > 0.98) without lag phases. The enantiomers of metalaxyl degraded rapidly, with half-lives ranging from only 0.8 to 1.5 days. Because of the faster degradation of S-metalaxyl, the enantiomeric ratio (ER) of metalaxyl is > 1 for both formulations and S increases with time. All forms of metalaxyl have higher degradation rates with the EC than with the WPC formulation in soil.
No significant change in soil pH was observed with fungicide application. Results suggest that the addition of fungicides led to a small change in the attributes of soil quality which, in overall, did not impact the eco-physiological status of the soil microbial community, as expressed by the availability of NH₄⁺ nitrogen and P.

Funding

Alexander von Humboldt Foundation and Gambrinus Fellowship of the University of Dortmund

Contact

Univ.-Prof. Dr. Dr. h.c. Michael Spiteller
M.Spiteller@infu.uni-dortmund.de
Phone: +49-(0)231-755-4080

Dr. rer. nat. Premasis Sukul
P.Sukul@infu.uni-dortmund.de
Phone: +49-(0)231-755-4089

Dr. rer. nat. Sebastian Zühlke
S.Zuehlke@infu.uni-dortmund.de
Phone: +49-(0)231-755-4088

Paper

1. Monkiedje, A.; Spiteller, M. (2006):
   "Soil ecotoxilogy and metabolism of metalaxyl and mefenoxan in tropical and temperate soils", In : Davis EB (Ed.) New developments in environmental research. Nova Science Publishers, Inc. NY ISBN 1-59454-629-0 295-346.

2. Monkiedje, A.; Spiteller, M.; Maniepi, S.J.N.; Sukul, P. (2007):
   "Influence of metalaxyl- and mefenoxam-based fungicides on chemical and biochemical attributes of soil quality under field conditions in a southern humid forest zone of Cameron" Soil Biology and Biochemistry, 39, 836-842.

3. Monkiedje, A.; Spiteller, M.; Maniepi, S.J.N.; Sukul, P. (2006):
   "The effect of land use on soil health indicators in peri-urban agriculture in the humid forest zone of southern Cameroon" Journal of Environmental Quality, 35, 2402-2409.

4. Monkiedje, A.; Zuehlke, S.; Maniepi, J. N.; Spiteller, M. (2007):
   "Elimination of racemic and enantioenriched metalaxyl based fungicides under tropical conditions in the field" Chemosphere, 69(4), 655-663.

5. Sukul, P.; Mazumder, A.; Spiteller, M. (2008):
   "Microbial population and their activities in soil as influenced by metalaxyl residues" Fresenius Environmental Bulletin, 17(1), 103-110.