The oxidation of ammonium is a key step in the nitrogen cycle, regulating the production of nitrate, nitrous oxide and dinitrogen. In marine and freshwater ecosystems, anaerobic ammonium oxidation coupled to nitrite reduction, termed anammox, accounts for up to 67% of dinitrogen production1,2,3. Dinitrogen production through anaerobic ammonium oxidation has not been observed in terrestrial ecosystems, but the anaerobic oxidation of ammonium to nitrite has been observed in wetland soils under iron-reducing conditions4,5. Here, we incubate tropical upland soil slurries with isotopically labelled ammonium and iron(iii) to assess the potential for anaerobic ammonium oxidation coupled to iron(iii) reduction, otherwise known as Feammox6, in these soils. We show that Feammox can produce dinitrogen, nitrite or nitrate in tropical upland soils. Direct dinitrogen production was the dominant Feammox pathway, short-circuiting the nitrogen cycle and resulting in ecosystem nitrogen losses. Rates were comparable to aerobic nitrification7,8 and to denitrification9, the latter being the only other process known to produce dinitrogen in terrestrial ecosystems. We suggest that Feammox could fuel nitrogen losses in ecosystems rich in poorly crystalline iron minerals, with low or fluctuating redox conditions.
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We appreciate discussions with M. K. Firestone and field and laboratory assistance from C. Torrens, A. C. McDowell, A. W. Thompson, T. Wood, M. Wong and M. Almaraz. We also thank R. Daly for allowing us to use to her anoxic gas station. This research was supported by grants DEB-0543558, DEB-0842385 and ATM-0628720 to W.L.S., DEB-0841993 to K.A.W., and DDIG 0808383 as well as a Graduate Research Fellowship to W.H.Y. from the US National Science Foundation. Additional support came from NSF grant DEB-0620910 to the Institute of Tropical Ecosystem Studies, University of Puerto Rico, and the International Institute of Tropical Forestry as part of the Long-Term Ecological Research Program in the Luquillo Experimental Forest. The International Institute of Tropical Forestry provided considerable infrastructural and technical support.
The authors declare no competing financial interests.
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Yang, W., Weber, K. & Silver, W. Nitrogen loss from soil through anaerobic ammonium oxidation coupled to iron reduction. Nature Geosci 5, 538–541 (2012). https://doi.org/10.1038/ngeo1530