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An experiment was performed to study non-target effects of difenoconazole (fungicide), deltamethrin (insecticide) and ethofumesate (herbicide) on microbial parameters in a clay-loam soil. Pesticides were applied as commercial formulations to soil samples at different concentrations (5, 50 and 500 mg kg(-1) DW soil) and then incubated under laboratory conditions for 3 months. Throughout the incubation period, microbial parameters were determined at days 7, 30, 60 and 90. At 5 mg kg(-1) DW soil, none of the three pesticides caused significant changes in soil microbial parameters. In contrast, at 500 mg kg(-1) DW soil, pesticide application decreased overall soil microbial activity, negatively affecting the activity of soil enzymes. Similarly, at 500 mg kg(-1) DW soil, difenoconazole and ethofumesate, but not deltamethrin, caused a pesticide-induced stress on soil microbial communities, as reflected by the respiratory quotient. Besides, deltamethrin and ethofumesate at 50 and 500 mg kg(-1) DW soil resulted in lower values of denitrification potential. It was concluded that, although pesticide concentration had a somewhat inconsistent and erratic effect on soil microbial parameters, pesticide application at 500 mg kg(-1) DW soil did have an impact on many of the microbial parameters studied here.

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The effects of deltamethrin, in the presence and absence of nitrate, on soil microbial activity (as reflected by the rates of soil microbial basal respiration, denitrification, and methanogenesis) were studied in a riparian wetland soil under both aerobic and anaerobic conditions. A microcosm study was carried out with soil collected from the vicinity of a wetland. The soil was then amended with 50, 125, and 250 mg deltamethrin kg j1 dry weight soil, in the presence and absence of 20 mg N-NO 3 j kg j1 dry weight soil. Half-life values for deltamethrin degradation ranged from 27 to 291 days, depending on experimental conditions. Nitrates had an inhibitory effect on deltamethrin degradation. Deltamethrin, under anaerobiosis, had an inhibitory effect on soil respiration; this effect was reversed in the presence of nitrate. An antagonistic effect between deltamethrin degradation and denitrification activity was observed. In the presence of nitrate, the activation of denitrifying bacteria led to competitive inhibition of methanogens. It was concluded that deltamethrin, designed to affect specific functions of its target organisms, also has an effect on nontarget organisms, that is, the soil microbial community. To our knowledge, this is the first report on the degradation and environmental impact of deltamethrin in a riparian wetland soil.

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