FREE AMMONIA INHIBITION OF NITRIFICATION IN RIVER SEDIMENTS LEADING TO NITRITE ACCUMULATION

Previous studies have reported that only 40% of nitrite (NO2-) in majo r rivers in Northern Ireland originates from land drainage. The remain der appears to originate from N transformations at the interface betwe en sediment and water within river systems. Streams in two major river catchments in Northern Ireland were monitored for the presence of NO2 -. Mean NO2- concentrations (which all exceeded 3.0 mu g N L-1 which i s the European Community (EC) guide values for supporting salmonid fis h) ranged from 3.8 to 84.5 mu g N L-1 in 14 streams in the Upper Bann and 10.7 to 60.0 mu g N L-1 in 14 streams in the Colebrooke river syst em. Evidence from multiple regression analysis suggested that NO2- acc umulation in these streams was associated with the presence of free NH 3. Incubation and differential N-15 labeling of sediment extracts with ammonium nitrate (NH4NO3) provided support for this hypothesis that s howed that the mechanistic process involved in NO2- accumulation is ni trification inhibition rather than denitrification. Free NH3 partially inhibits NO2- oxidation by Nitrobacter, but does not inhibit the oxid ation of ammonium (NH4+) by Nitrosomonas. Most of the NH4+ in streams originates from pollution by agricultural effluents and must be curtai led to prevent NO2- accumulation.