Free Ammonia Inhibition of Nitrification in River Sediments Leading to Nitrite Accumulation

Abstract

Abstract Previous studies have reporled that only 40% of nitrite (NO − 2 ) in major rivers in Northern Ireland originates from land drainage. The remainder appears to originale from N transformations at the interface between sediment and water within river systems. Streams in two major river catchments in Northern Ireland were monitored for the presence of NO − 2 . Mean NO 2 concentrations (which all exceeded 3.0 µg N L −1 which is the European Community (EC) guide values for supporting salmonid fish) ranged from 3.8 to 84.5 µg N L −1 in 14 streams in the Upper Bann and 10.7 to 60.0 µg N L −1 in 14 streams in the Colebrooke river system. Evidence from multiple regression analysis suggested that NO 2 accumulation in these streams was associated with the presence of free NH 3 . Incubation and differential 15 N labeling of sediment extracts with ammonium nitrale (NH 4 NO 3 ) provided support for this hypothesis that showed that the mechanistic process involved in NO − 2 accumulation is nitrification inhibition rather than denitrification. Free NH 3 partially inhibils NO − 2 oxidation by Nitrobacter , but does not inhibit the oxidation of ammonium (NH + 4 ) by Nitrosomonas . Most of the NH + 4 in streams originates from pollution by agricultural effluents and must be curtailed to prevent NO − 2 accumulation.