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

Summary 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 12.33 t 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. Highlights Feammox was investigated by ammonium 15 N labelled isotopic tracing technique. Feammox rates in mangrove were larger than those in mud flats without mangrove. The nitrogen loss contribution of Feammox in mangrove was less than in other ecosystems. Tidal fluctuations and the large TOC would accelerate the Feammox process.