The Role of Sediments in Modulating Nitrous Oxide Production in the Southern Benguela Upwelling System: Insights From Stable Isotopic Tracers

Abstract

The ocean accounts for ∼20%–30% of global nitrous oxide (N₂O) emissions, with coastal upwelling systems estimated to contribute disproportionately to the sea-air flux of this potent greenhouse gas. To investigate the mechanisms of and controls on N₂O production in coastal upwelling systems, we measured the concentration and nitrogen and oxygen isotopic composition of N₂O (δ¹⁵N-N₂O and δ¹⁸O-N₂O) along a cross-shelf transect in the Southern Benguela Upwelling System (SBUS). At the shelf bottom, N₂O concentrations increased from the outer shelf toward the shore (11–32 nM) inversely to dissolved oxygen (182 ± 17 to <1 μM) and in concert with the remineralization tracers, apparent oxygen utilization (108 ± 21 to 221 ± 33 μM) and nitrogen (N)-deficit (up to 20.4 μM). These observations suggest that both nitrification and denitrification may be involved in N₂O production on the SBUS shelf. The δ¹⁵N-N₂O implicates both processes as potential N₂O sources on the shelf, with high δ¹⁸O-N₂O values (≤57.2‰) specifically incriminating sediments as the primary N₂O source to the water column. Isotopic changes across the shelf delineate three discrete domains with distinct N₂O sources. Sedimentary nitrification and/or denitrification dominate N₂O production on the midshelf, while coupled nitrification-denitrification explains N₂O production on the inner-shelf. At the shallow inner-shelf where oxygen is depleted, both water column and sedimentary denitrification account for the production and partial consumption of N₂O. This study illuminates the disproportionate contribution of sedimentary N cycling to N₂O production on the SBUS shelf.