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 (N 2 O) 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 N 2 O production in coastal upwelling systems, we measured the concentration and nitrogen and oxygen isotopic composition of N 2 O (δ 15 N‐N 2 O and δ 18 O‐N 2 O) along a cross‐shelf transect in the Southern Benguela Upwelling System (SBUS). At the shelf bottom, N 2 O 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 N 2 O production on the SBUS shelf. The δ 15 N‐N 2 O implicates both processes as potential N 2 O sources on the shelf, with high δ 18 O‐N 2 O values (≤57.2‰) specifically incriminating sediments as the primary N 2 O source to the water column. Isotopic changes across the shelf delineate three discrete domains with distinct N 2 O sources. Sedimentary nitrification and/or denitrification dominate N 2 O production on the midshelf , while coupled nitrification‐denitrification explains N 2 O 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 N 2 O. This study illuminates the disproportionate contribution of sedimentary N cycling to N 2 O production on the SBUS shelf.