The objective of the present study was to study mechanisms controlling substrate consumption and growth of ammonia-oxidizing bacteria (AOB) in a NH3 loaded agricultural biotrickling air filter. A series of filter NH3 load manipulations was performed. HNO2 was observed to constitute around half of the AOB oxidation products, and assumed the main inhibitor of AOB growth and substrate consumption. Under operating conditions with stable NH3 load, biomass increased until self-inhibition restricted further growth. Sudden load changes affected pH and the level of free HNO2, and caused an immediate (within few hours) change in substrate consumption rate. At persisting load change (over a period of weeks) the biomass adapted to a new level dictated by the new average NH3 load. It is suggested that the theoretical minimum outlet NH3 concentration is controlled by the filter water concentrations of NH4+ and E(NO2- + NO3-), which due to conservation of charge balance is found in an approximately 1:1 ratio. Heterotrophic oxidation of odorous compounds appeared to be unaffected by short-term changes in AOB inhibition levels. (C) 2010 Elsevier B.V. All rights reserved.