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

Abstract

On the basis of in situ NO3- 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- profiles horizontally separated by 12 mm reflected highly variable NO3- penetration depths, NO 3- consumption rates, and nitrification. On average the turnover time of the pore-water NO3- 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- (1.44 mmol m-2 d-1) that balanced the N 2 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- stored inside foraminifera represented ,80% of the total benthic NO3- pool, the slow intracellular NO3- 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. © 2009, by the American Society of Limnology and Oceanography, Inc.