Nitrogen removal by denitrification and annamox processes in freshwater rivers of high nitrogen loading region of China

The importance of anaerobic ammonium oxidation (anammox) – a metabolic pathway that can generate dinitrogen – remains poorly understood in freshwater river system. Using the 15N-isotope pairing technique (15N IPT) combined with membrane inlet mass spectrometry (MIMS), the potential rates of denitrification, anammox and total N removal, and their respective contributions to total N2 production were evaluated in 11 rivers in the Taihu Lake region of China. The measured potential rates of denitrification, total N removal and anammox varied from 18.9±0.3 to 70.0±11.4, 26.3±0.4 to 71.3±11.1 and 1.3±0.3 to 11.0±2.5 μmol N m-2 h-1, respectively. The relative contribution of anammox to total N2 production (ra%) ranged from 2.0±0.8% to 29.9±0.7%. The mean potential denitrification and the total N removal rates varied spatially in these 11 rivers, with the highest rates occurring in the western rivers of the region, while the mean potential anammox rates and ra% displayed the opposite trend with the highest values occurring in the southern rivers of the region. The contents of nitrate and dissolved organic carbon in sediments appeared to be the primary controlling factors for denitrification and anammox in these studied rivers. Our results indicated that the potential rates of N removal varied spatially, and denitrification is the dominant activity for removing fixed N but the role of anammox is not negligible in freshwater rivers.