POTENTIAL DENITRIFICATION AND NITRIFICATION TESTS FOR EVALUATION OF PESTICIDE EFFECTS IN SOIL

The acute toxicity effects of 54 pesticides on denitrification and nit rification in soil were studied in the laboratory. Potential denitrifi cation activity (PDA) was assayed with an acetylene inhibition method, during which the specific growth rate constant of denitrifiers (mu(PD A)) could also be determined. Potential ammonium oxidation activity (P AO) was assayed with the chlorate inhibition method. A first screening test of the pesticides at 100 mu g active ingredient (a.i.) g(-1) dry soil (d.s.), revealed that 23% had a significant effect on PDA, 26% o n mu(PDA) and 35% on PAO. For denitrification both stimulatory (8-11% of the pesticides) and inhibitory (15%) effects were observed, while e ffects on nitrification were all inhibitory. In a subsequent dose-resp onse test NOEC, EC50 and EC90 were determined for mancozeb (PDA and PA O) and 2,4-DB, ioxynil, maneb, zineb and nitrapyrin (PAO). The NOEC of mancozeb were 1.0 and 0.4 mu g a.i. g(-1) (d.s.) for PDA and PAO, res pectively, and the corresponding EC50 were 7.1 and 2.1 mu g a.i. g(-1) respec-tively. The methods emphasize short incubation periods with re latively short intervals between determination of product formation, t hus allowing quantitative measure-ments with kinetic considerations. I n addition to being rapid and sensitive, PDA, mu(PDA) and PAO reflect different parts of the bacterial biomass and hence complement each oth er in the interpretation of test results. Therefore, all three paramet ers are suitable for inclusion in a test system for assessing effects on the soil environment.