Soil quality changes resulting from the application of the fungicides mefenoxam and metalaxyl to a sandy loam soil

Experiments were conducted to investigate soil quality changes resulting from a single application of selected fungicides at the following rates 2, 10, 20, 200 and 1000 μg g−1 dry soil and 1, 5, 10, 100, 1000 μg g−1 dry soil and for metalaxyl and mefenoxam its R-enantiomer, respectively. Microbial activity indices (available N, ammonification and nitrification rates, microbial population density and specific enzymatic systems) were the more sensitive indicators of change. The values of these indicators generally increased with the period of incubation and decreased or increased at high rates. Mefenoxam and metalaxyl and their major acid metabolites were toxic to soil micro-organisms at high doses with 16 h-IC50 values of 4500, 12,500, and 15,800 μg g−1 soil (dry weight), respectively. Thus, their ranking in terms of acute toxic effects was in the order mefenoxam>metalaxyl>acid metabolite. Incorporation of the fungicides in soil resulted in increased ammonium and nitrate levels. These changes stimulated soil microbial activity as evidenced by increased ammonification and nitrification rates, increased total viable bacteria counts especially after short-term exposure. Of the enzyme activities studied, dehydrogenase activity was the most sensitive to fungicides. This sensitivity was more pronounced with metalaxyl on long-term exposure. In general, the stimulation of nitrogen transformation and bacterial density were more pronounced with mefenoxam.