Co-occurrence of aerobic ammonia oxidation, anaerobic ammonia oxidation and nitrite oxidation in oxic riverbeds and their relationships with net nitrification efficiency

Abstract

In short term, re-mineralized nitrogen as ammonia can either be conserved in an ecosystem through its complete oxidation to nitrate e.g. ‘efficient nitrification’ or lost via oxidation to N2 gas e.g., ‘inefficient nitrification’. Here, 15N tracers and molecular analyses were used to characterize the ‘nitrification efficiency’ in relation to aerobic ammonia oxidation, anaerobic ammonia oxidation (anammox) and nitrite oxidation across a range of oxic riverbeds. Here 15NO2- from 15NH4+ was rapidly consumed by anammox/denitrification and/or nitrite oxidation and although the total rate of ammonia oxidation (i.e. 15NO3- + 15N2) was conserved, nitrification efficiency varied from 22.2% to 99.7%. Nitrification efficiency was highest where the contribution from anammox to N2 production (ra) was lowest, and maximal where anammox was absent, suggesting competition between nitrite oxidation and anammox for nitrite. Nitrification efficiency was also highest where the abundance of nxrB gene (Nitrospira + Nitrobacter) was greatest, along with the highest abundance of comammox Nitrospira amoA gene. These results reveal a gradient in riverbed nitrification efficiency that was related primarily to Nitrospira dominating the nitrite oxidizing bacteria (NOB). A preincubation of soluble reactive phosphorus (SRP) selectively increased the degree of nitrification efficiency in some riverbed sediments by stimulating nitrite oxidation. Furthermore, anammox was more important where the abundance of hzsB and amoA genes were greatest, indicating an interaction between aerobic and anaerobic ammonia oxidation. In these oxic riverbeds, aerobic ammonia oxidizing microorganisms (AOM) and anammox bacteria may aggregate together, AOM consumed oxygen at a rate of 22.28 nmol g-1 h-1 and competed with heterotrophic respiration for 8% of the total oxygen consumption, making oxygen limited in the aggregates and thus enabling anammox. Furthermore, the aerobic ammonia oxidation can provide nitrite to sustain the anammox bacteria.