Direct determination of nitrogen cycling rates and pathways in Arctic fjord sediments (Svalbard, Norway)
Limnol. Oceanogr., 55(2), 2010, 740-752 | DOI: 10.4319/lo.2010.55.2.0740
ABSTRACT: Results from a study of benthic nitrogen cycling in two Arctic fjords are presented. Intact sediment core incubations were used to quantify net fluxes of dissolved inorganic nitrogen, organic nitrogen, organic carbon, and oxygen across the sediment-water interface. Rates of gross denitrification, anammox, nitrification, dissimilatory nitrate reduction to ammonium, and N2 fixation were quantified using core incubations and slurry experiments. Profiles of dissolved inorganic nitrogen in pore-water, and organic carbon and nitrogen contents in the solid phase, were also obtained. Net nitrogen losses as N2, measured directly as changes in N2 : Ar, ranged from 152 µmol N m-2 d-1 to 453 µmol N m-2 d-1. Gross denitrification, as determined using 15N-NO3- tracers, was 34-294 µmol N m-2 d-1. Rates of anammox ranged from 10 µmol N m-2 d-1 to 26 µmol N m-2 d-1 and contributed 5-23% of gross N2 generation. Nitrification rates were as high as 833 µmol N m-2 d-1 and sediments were a substantial source of nitrate to the water column (169-393 µmol N m-2 d-1 efflux). Uptake of ammonium (52-87 µmol N m-2 d-1), dissolved organic nitrogen (291-486 µmol N m-2 d-1), and dissolved organic carbon (1313-2504 µmol N m-2 d-1) were observed. Nitrogen loss from the seafloor is shown to occur at rates comparable to temperate coastal environments and the sediments provide a substantial source of nitrate to the water column of Arctic fjords. In addition, anammox is confirmed as an important pathway in Arctic marine sediments, although N2 production is dominated by denitrification.