Nitrogen removal in wastewater treatment is energy consuming and often carried out in biofilm nitrifying trickling filters (NTFs). We investigated nitrification potential and population dynamics of nitrifying bacteria in pilot-plant NTFs fed with full-scale plant wastewater with high (8-9 mg NH4+ l(-1)) or low (<0.5 mg NH4+ l(-1)) ammonium concentrations. After ammonium shifts, nitrification potentials stabilized after 10-43 days depending on feed regime. An NTF fed with 3 days of high, and 4 days of low load per week reached a high nitrification potential, whereas a high load for 1 day a week gave a low potential. Nitrosomonas oligotropha dominated the AOB and changes in nitrification potentials were not explained by large population shifts to other AOBs. Although nitrification potentials were generally correlated with the relative amounts of AOB and NOB, this was not always the case. Ammonium feed strategies can be used to optimize wastewater treatment performance. (C) 2011 Elsevier Ltd. All rights reserved.