Nitrogen cycling in a deep ocean margin sediment (Sagami Bay, Japan)

On the basis of in situ NO 3 −1 microprofiles and chamber incubations complemented by laboratory‐based assessments of anammox and denitrification we evaluate the nitrogen turnover of an ocean margin sediment at 1450‐m water depth. In situ NO 3 −1 profiles horizontally separated by 12 mm reflected highly variable NO 3 −1 penetration depths, NO 3 −1 consumption rates, and nitrification. On average the turnover time of the pore‐water NO 3 −1 pool was ~0.2 d. Net release of NH 4 + during mineralization (0.95 mmol m −2 d −1 ) sustained a net efflux of ammonia (53%), nitrification (24%), and anammox activity (23%). The sediment had a relatively high in situ net influx of NO 3 −1 (1.44 mmol m −2 d −1 ) that balanced the N 2 production as assessed by onboard tracer experiments. N 2 production was attributed to prokaryotic denitrification (59%), anammox (37%), and foraminifera‐based denitrification (4%). Anammox thereby represented an important nutrient sink, but the N2 production was dominated by denitrification. Despite the fact that NO 3 −1 stored inside foraminifera represented ~80% of the total benthic NO 3 −1 pool, the slow intracellular NO 3 −1 turnover that, on average, sustained foraminifera metabolism for 12‐52 d, contributed only to a minor extent to the overall N 2 production. The microbial activity in the surface sediment is a net nutrient sink of ~1.1 mmol N m −2 d −1 , which aligns with many studies performed in coastal and shelf environments. Continental margin areas can act as significant N sinks and play an important role in regional N budgets.