Nitrification under changing salinities (0-9%), temperatures (6-50 degrees C), ammonia (0-5 g N L(-1)) and nitrite concentrations (0-0.4 g N L(-1)) was investigated in fixed-bed reactors. For all conditions ammonia oxidation rates (AOR) were lower than nitrite oxidation rates (NOR). AORs and NORs increased from 12.5 to 40 degrees C and were very low at 6 degrees C and almost zero at 50 degrees C. No recovery of nitrification was obtained after incubation at 50 degrees C, whereas nitrification was restorable after incubation at 6 degrees C. Ammonia concentrations of 5 g N L(-1) or nitrite concentrations up to 0.125 g N L(-1) decreased AOR to almost zero. AORs and NORs recovered if ammonia or nitrite was removed. At concentrations of 1 and 5 g N L(-1) ammonia AOR and NOR were inhibited by 50%, whereas 27 mg N/L nitrite inhibited AOR by 50%. (C) 2011 Elsevier Ltd. All rights reserved.