Monday, August 3, 2009 - 1:30 PM

COS 3-1: Feammox: A novel pathway for ammonium oxidation and nitrogen loss from terrestrial ecosystems

Whendee L. Silver1, Wendy Yang1, and Karrie A. Weber2. (1) University of California, Berkeley, (2) University of Nebraska

Background/Question/Methods We report on a novel pathway in the terrestrial nitrogen (N) cycle that couples anaerobic ammonium oxidation with iron(III) reduction. The pathway, called Feammox, results in the production of dinitrogen (N2) gas or nitrite (NO2-). Nitrite produced via Feammox can be subsequently reduced to nitrous oxide (N2O) gas, N2, or ammonium (NH4+) via dissimilatory NO2- reduction or abiotic oxidation of iron(II). We conducted three experiments. As a general test of anaerobic NH4+ oxidation we added equimolar 15NH4+ and natural abundance NO2- (14NO2-) to iron (Fe)-rich tropical forest soils. Acetylene (C2H2) was added as an inhibitor of N2O reduction to N2 and of anammox to a second set of isotopically labeled samples. This method can distinguish direct N2 production via Feammox or anammox from N2 produced from denitrification of Feammox-generated NO2-. Changes in the 15-14N2 mole fraction could result from anammox or Feammox, whereas changes in the 15-15N2 mole fraction indicated Feammox. In the presence of C2H2, increases in the 15N enrichment of N2 suggest direct N2 production via Feammox. In the second experiment we added NH4+ only or NH4+ with Fe(III) to pre-incubated anaerobic slurries to test for NO2- production. We explored the potential for Feammox to produce N2 in a third experiment. Pre-incubated soil slurries were amended with 15NH4+ (24 h only) or 15NH4+ with Fe(III) (6 and 24 h) in stoichiometric equivalency. Dinitrogen was measured immediately after collection on an isotope ratio mass spectrometer.

Results/Conclusions

There was no 15-14N2 produced during the 96 h incubation with or without C2H2 added. However, we measured considerable 15-15N2 production. Acetylene addition (24 h only) increased the 15-15N2 mole fraction suggesting that some N2 was produced directly via Feammox and did not result from anammox or biological denitrification of NO2-. In the second experiment, aqueous NO2- concentrations were significantly higher in samples that had received NH4+ and Fe(III) than soils that had received only NH4+ after 22 hours. A similar trend occurred at 3 h, but the differences were not statistically significant. In the third experiment, we measured significantly higher 15-15N2 mole fraction in the 15NH4+ and Fe(III) treatment than in the 15NH4+ only or blank (soil free) treatments. Using mass balance calculations we determined that over 20% of the added NH4+ was lost as N2 via Feammox. Our results suggest that Feammox occurs in these upland soils, and has the potential to produce significant N2 and NO2-.