Adolphe Monkiedje

Effects of the phenylamide fungicides, mefenoxam and metalaxyl, on the microbiological properties of a sandy loam and a sandy clay soil

Abstract. The objective of the study was to compare ecotoxicological data obtained from laboratory experiments on the side-effects of three phenylamide fungicides, pure metalaxyl (racemic mixture of R- and S-enantiomers), formulated metalaxyl and mefenoxam (only active R-enantiomer) on the chemical and biochemical parameters of two soils of different type and origin. The purpose of the comparison was to determine to what extent mefenoxam, developed as alternative to metalaxyl, can affect the activity of soil micro-organisms and their processes, and to elucidate the differences between the effects of pure and formulated metalaxyl. The dynamics of the quantitative changes in biochemical parameters induced by the addition of these fungicides at their recommended field rate were determined in a sandy clay soil from Cameroon and a sandy loam soil from Germany, during a 120-day incubation experiment. The type of soil significantly influenced the effect of these fungicides on the soil parameters studied. Incorporation of these fungicides generally stimulated the activity of phosphatases and ß-glucosidase, mineralization and the availability of N and most plant nutrients in soils. The activity of dehydrogenase and the availability of NO3– were generally adversely affected. Among the fungicides tested, the stimulation was more pronounced with mefenoxam followed by formulated metalaxyl.

Sorptive behavior of the phenylamide fungicides, mefenoxam and metalaxyl, and their acid metabolite in typical Cameroonian and German soils

Laboratory soil sorption experiments were conducted on mefenoxam, formulated metalaxyl (F-metalaxyl), pure metalaxyl (P-metalaxyl) and metalaxyl acid metabolite to elucidate differences in their sorptive behaviour on typical Cameroonian forest soil (sand clay loam, pH 4.8 and 3.01% OC) and German soil (sandy loam, pH 7.2, 1.69% OC) using a batch equilibrium method. The data obtained on all test chemicals conformed to linear and Freundlich adsorption isotherms. The Langmuir equation failed to describe the sorption of the substances tested. All substances were adsorbed to a greater extent by the Cameroonian soil. The average percentage adsorptions for mefenoxam, F-metalaxyl, P-metalaxyl and the acid metabolite on the Cameroonian soil were 27.8%, 28.3%, 31.8% and 46.8% respectively while for the German soil they were 21.7%, 21.5%, 24.7% and 9.8% respectively. The KD and KF parameters and the Freundlich exponential term (1/n) were low, indicating that the interactions between soil particles and the fungicides were weak. The sorption parameters were lower in the German soil. P-metalaxyl exhibited a higher adsorption capacity than F-metalaxyl in both soils. Mefenoxam and F-metalaxyl exhibited similar sorption parameters in soils, whereas those of P-metalaxyl and acid metabolite differed. Differences observed in the adsorption between the two soils could be attributed to their properties. Desorption studies revealed that the adsorbed fungicides were not firmly retained by soil particles and their adsorption was reversible. Desorption of adsorbed mefenoxam, P-metalaxyl and of the acid metabolite from German soil was almost completely reversible with percentage desorption rates of more than 91.0%, whereas the rate for F-metalaxyl was 74.1%. All compounds exhibited some resistance to desorption from the Cameroonian soil, with percentage desorption rates less than 77.0%. Therefore if degradation in the soil is slow the fungicides described have a potential to leach to lower soil horizons.

Assessment of the acute toxic effects of the fungicide Ridomil plus 72 on aquatic organisms and soil micro-organisms

The acute toxic effects of Ridomil plus 72 (Ridomil), used as a fungicide in agriculture, were studied under laboratory conditions. These effects on freshwater cladoceran (Chydorus eurynotus) and freshwater fish (Oreochromis niloticus) were determined using standard bioassay procedures. The 4 h LC50 for Chydorus eurynotus was 6.9 mg/L and the 96 h LC50 for Oreochromis niloticus was 1.1 mg/L. The toxicity of Ridomil to a mixed population of soil micro‐organisms was measured using oxygen uptake and growth as measured by turbidity. At short‐term exposure (48 min), Ridomil was moderately toxic to the culture at 5000 mg/L and above, based on an activity quotient (AQ) of 0.50–0.70. For longer exposure periods (up to 6 h), Ridomil was slightly toxic to the culture at 200 to 500 mg/L. At 1000 mg/L, Ridomil was moderately toxic and at 3000 mg/L and above, Ridomil was extremely toxic to soil microorganisms. The toxicity of metalaxyl (one of the two active ingredients of Ridomil) to these micro‐organisms was measured using growth as measured by the turbidity change. The average toxic endpoint (16 h IC50) was 1100 mg/L. The acute toxic values of Ridomil found in these studies were much below the expected environmental concentrations resulting from normal applications of the chemical on a cocoa tree as a fungicide. ©2000 John Wiley & Sons, Inc. Environ Toxicol 15: 65–70, 2000