Controls on N2 production via iron reduction coupled to anaerobic ammonium oxidation

Iron (Fe) reduction coupled to anaerobic ammonium (NH4+) oxidation is a novel nitrogen (N) cycling pathway that can lead to ecosystem N loss via production of dinitrogen (N2), nitrate (NO3-), or nitrite (NO2-). This pathway, termed Feammox, can short circuit the N cycle via direct N2 production or lead to N2O and N2 production via denitrification of Feammox-generated NO2- and NO3-. Theoretically, Feammox becomes less thermodynamically favorable as pH increases, with pH 6.5 as the threshold for favorability of Feammox to NO2- or NO3-. Availability of iron oxides may also limit Feammox rates because high labile C availability drives high Fe reduction rates under anaerobic soil conditions. In contrast, NH4+ availability may not be a strong control on Feammox rates if gross mineralization and/or dissimilatory NO3- reduction to NH4+ continue to produce NH4+ under anaerobic conditions. We performed laboratory experiments using surface soils (0-10 cm depth) from the Luquillo Experimental Forest, Puerto Rico to investigate the controls on Feammox rates. Soil slurries were pre-incubated in an oxygen (O2)-free glove box for 6 days to deplete background O2, NO2-, and NO3-. We measured the 30N2 mole fraction of produced N2 at 24 hours after the addition of either 15NH4+ alone or 15NH4+ in stoichiometric equivalency with an amorphous Fe(III) gel (HFO) to the soil slurries (n = 8). Feammox rates were conservatively estimated from 30N2 alone because 30N2 production could result only from Feammox of 15NH4+ whereas 29N2 production could result from a variety of pathways. In soils at pH 4.27 ± 0.02, we measured rates of Feammox ranging from 0.32 ± 0.13 μg N g-1 d-1 (± SE), following 15NH4+ addition alone, to 1.20 ± 0.28 μg N g-1 d-1 with the addition of both 15NH4+ and Fe(III). In soils at pH 6.12 ± 0.03, Feammox rates ranged from 0.03 ± 0.01 μg N g-1 d-1, following 15NH4+ addition alone, to 0.02 ± 0.01 μg N g-1 d-1 with the addition of both 15NH4+ and Fe(III). Our data suggest that the threshold for thermodynamic favorability of Feammox may be lower than calculated (~6.2) and that the Fe oxide limitation of Feammox rates is less important at high pH. Feammox is most likely to occur in highly weathered soils rich in poorly crystalline Fe that experience fluctuating redox conditions so that Fe oxides are replenished and relatively low pH conditions are restored during oxic periods.