Nitrogen loss through anaerobic ammonium oxidation coupled with iron reduction in a mangrove wetland

Anaerobic ammonium oxidation coupled with iron(III) reduction (Feammox) with dinitrogen, nitrite or nitrate as end-products is the most recently discovered nitrogen cycling process. This reaction has been observed in tropical forest soils, paddy soils and intertidal wetlands. However, Feammox has not been measured in mangrove wetlands. In this study, sediment slurry incubation experiments were combined with isotope tracing and acetylene inhibition techniques. Feammox was detected in mangrove sediments and bare flats' (mud flats without mangrove), with potential rates of 0.48 (0.03 SE) mg N kg(-1)day(-1) (accounting for 6.4% of the total nitrogen loss through N-2) and 0.38 (0.02 SE) mg N kg(-1)day(-1) (accounting for 6.7% of the total nitrogen loss through N-2), respectively. Microbially reducible iron(III) was added, which significantly (P<0.01) increased the Feammox rate in contrast to no addition of iron(III). It was estimated that a loss of 12.33t N year(-1) was associated with Feammox in mangrove sediments of the Jiulong River Estuary, accounting for 0.04% of the total external inorganic nitrogen transported into the estuary. Overall, these findings demonstrate that Feammox can act as a nitrogen loss mechanism in mangroves.