N 2 production through denitrification and anammox across the continental margin (shelf–slope–rise) of the Ulleung Basin, East Sea

Abstract

Experimental determinations of nitrogen cycling in deep‐sea sediments are strongly underrepresented in the databases. To investigate the total N 2 production rates and relative contribution of denitrification and anaerobic ammonium oxidation (anammox) to benthic fixed‐N removal processes, we conducted 15 N isotope‐labeling incubation experiments in whole cores and slurries at nine stations across the continental margin from the shelf ( 2000 m) Ulleung Basin (UB) in the East Sea. The total N 2 production rates (anammox plus denitrification) in the center of the UB (8.4 ± 0.2 μmol N m −2 h −1 ) were high compared to most other deep‐sea sediments at similar water depths. Denitrification rates decreased from the shelf (7.6 ± 0.6 μmol N m −2 h −1 ) to the basin (3.2 ± 0.4 μmol N m −2 h −1 ), in proportion to benthic oxygen consumption, whereas anammox rates remained relatively constant or even increased slightly (1.3–4.1 μmol N m −2 h −1 ). The contribution of anammox to the total N 2 production ( ra ) increased with increasing water depth from the shelf (ca. 17%) to the basin (ca. 56%). The enhanced ra in the center of the UB was associated with an increased availability of nitrite for anammox, which was likely a result of the competitive suppression of denitrification by manganese reduction under MnO 2 ‐rich conditions. Our results emphasize the importance of anammox as a sink for reactive nitrogen in deep‐sea sediments and contribute toward a mechanistic understanding of the factors controlling benthic reactive nitrogen loss in the ocean.

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