N2 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 N2 production rates and relative contribution of denitrification and anaerobic ammonium oxidation (anammox) to benthic fixed-N removal processes, we conducted 15N isotope-labeling incubation experiments in whole cores and slurries at nine stations across the continental margin from the shelf (˂ 200 m) and into the deep (˃ 2000 m) Ulleung Basin (UB) in the East Sea. The total N2 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 N2 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 MnO2-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.