Saltzman, S., Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet Dagan 20-500, Israel
Brates, N., Institute of Soils and Water, Agricultural Research Organization, The Volcani Center, Bet Dagan 20-500, Israel

The effects of four pesticides and CO2 stress conditions on the oxidation rate of ammonium in a sandy soil was studied with incubation experiments. At initial concentrations of 1–10 ppm the pesticides tested did not affect the oxidation rate under optimal incubation conditions, but had a slight inhibitory effect under C02stress conditions. The amplitude of the maximal depression in the activity of the ammonium oxidizers (AMD) was 25-30%, and the time required for the restitution of 90% of the normal microbial activity (TD90) varied between 3 and 12 days. At the high application dose (100 ppm) the AMD due to pesticides only was 37-46%, and the TD90was up to 5 days. Under the combined effect of pesticides and C02stress the AMD was 53-70%, and the time deficit was 13–14 days for metolachlor and EPTC, and 59–64 days for chlorpyrifos and fenamiphos. In the last case, the effects of the chemical and CO stresses on the oxidation rate of ammonium were synergistic. © 1990, Marcel Dekker, Inc.