Experiments based on oxygen uptake measurements were developed to ensure that azide (99% pure; Merck, Darmstadt, Germany) and allylthiourea (97% pure; Rhône-Poulenc Ltd., Manchester, United Kingdom) completely and instantaneously inhibited nitrite and ammonia oxidizers, respectively, without affecting other activities. Oxygen uptake measurements were obtained with a batch type respirometer (model 5300 biological oxygen monitor; Yellow Springs Instruments, Yellow Springs, Ohio) at 20°C and pH 7.6. Washed biomass was added to two incubation chambers filled with 10 ml of oxygen-saturated mineral medium (14).