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

Glud, Ronnie N., Bo Thamdrup, Henrik Stahl, Frank Wenzhoefer, Anni Glud, Hidetaka Nomaki, Kazumasa Oguri, Niels Peter Revsbech, Hiroshi Kitazato

Limnol. Oceanogr., 54(3), 2009, 723-734 | DOI: 10.4319/lo.2009.54.3.0723

ABSTRACT: On the basis of in situ NO3-1 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 NO3-1 profiles horizontally separated by 12 mm reflected highly variable NO3-1 penetration depths, NO3-1 consumption rates, and nitrification. On average the turnover time of the pore-water NO3-1 pool was ~0.2 d. Net release of NH4+ 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 NO3-1 (1.44 mmol m-2 d-1) that balanced the N2 production as assessed by onboard tracer experiments. N2 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 NO3-1 stored inside foraminifera represented ~80% of the total benthic NO3-1 pool, the slow intracellular NO3-1 turnover that, on average, sustained foraminifera metabolism for 12-52 d, contributed only to a minor extent to the overall N2 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.

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