The role of sediments in modulating nitrous oxide production in the Southern Benguela Upwelling System: insights from stable isotopic tracers

The ocean accounts for ~20 to 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 resolve 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. At the shelf bottom, N₂O concentrations increased from the outer shelf towards the shore (11 to 32 nM) inversely to dissolved oxygen (182 ± 17 to <1 μM) and in concert with the remineralization tracers, Apparent Oxygen Utilization (AOU; 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 confirms both processes as potential N₂O sources on the shelf, with high δ¹⁸O-N₂O values (≤ 57.2‰) specifically implicating the sediments as the primary N₂O source to the water column. Isotopic changes across the shelf delineate three discrete domains, each with distinct N₂O sources. Sedimentary nitrification dominates N₂O production on the midshelf, while coupled nitrification-denitrification or direct denitrification explains N₂O production on the inner-shelf. At the shallow inner-shelf, where oxygen concentrations are depleted, both water column and sedimentary denitrification account for the production and partial consumption of N₂O. This study uncovers the disproportionate contribution of  sedimentary N cycling to N₂O production on the SBUS shelf.