Print PageIron reduction coupled to anaerobic ammonium oxidation (Feammox) is a biogeochemical pathway that short circuits the terrestrial nitrogen (N) cycle, resulting in soil N loss. Feammox can produce NO3- or NO2-, which can be denitrified to nitrous oxide or dinitrogen (N2), or it can produce N2 directly, thus avoiding the production of reactive N gases. We quantified rates of Feammox in soils (0-10 cm depth) from a humid tropical forest and a temperate grassland. We added either 15NH4+ alone or 15NH4+ in stoichiometric equivalency with an amorphous Fe(III) gel (HFO) to soil slurries (n = 8) anoxically pre-incubated for 6 days to deplete background O2, NO2-, and NO3-. We measured 30N2 mole fractions of produced N2 after 6, 12, and 24 hours. 30N2 production could result only from Feammox of 15NH4+.In a second experiment with the tropical forest soil, we added acetylene (C2H2), an inhibitor of N2O reduction to N2 and of anammox, to a second set of isotopically labeled samples to distinguish direct N2 production via Feammox or anammox from N2 produced from denitrification of Feammox-generated NO2- or NO3-.
Results/Conclusions
In both tropical forest and temperate grassland soils, 30N2 production was significantly greater in the 15NH4+ and Fe(III) treatment than the 15NH4+ treatment, suggesting that Feammox is stimulated by Fe additions. Rates were also greatest at 6 hours, suggesting that Feammox responds rapidly to the addition of reactive Fe(III). In a second experiment with the tropical forest soil, 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). The presence of C2H2 decreased 30N2 production (p < 0.08), indicating that 45 % of 30N2 loss resulted from Feammox to NO2- or NO3-. Our results suggest that Feammox can occur in a wide range of upland soils, contributing to ecosystem N loss via NO2-, NO3-, and N2 production.
