Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

International audienceThe biogeochemical mechanism of bacterial N2O production in the ocean has been the subject of many discussions in recent years. New isotopomeric tools can help further knowledge on N2O sources in natural environments. This research shows and compares hydrographic, nitrous oxide concentration, and N2O isotopic and isotopomeric data from three stations across the South Pacific Ocean, from the center of the subtropical oligotrophic gyre (~26° S; 114° W) to the upwelling zone along the central Chilean coast (~34° S). Althought AOU/N2O and NO3- trends support the idea that most of N2O source (mainly from intermediate water (200–1000 m)) come from nitrification, N2O isotopomeric composition (intramolecular distribution of 15N isotopes in N2O) reveals an abrupt change in the mechanism of nitrous oxide production, always observed through lower SP (site preference of 15N), at a high – stability layer, where particles could act as microsites and N2O would be produced by nitrifier denitrification (reduction of nitrite to nitrous oxide mediated by primary nitrifiers). There, nitrifier denitrification can account for 40% and 50% (center and east border of the gyre, respectively) of the nitrous oxide produced in this specific layer. This process could be associated with the deceleration of sinking organic particles in highly stable layers of the water column. In constrast, coastal upwelling system is characterized by oxygen deficient condition and some N deficit in a eutrophic system. Here, nitrous oxide accumulates up to 480% saturation, and isotopic and isotopomer signal show highly complex nitrous oxide production processes, which presumably reflect both the effect of nitrification and denitrification at low oxygen levels on N2O production, but non N2O consumption by denitrification was observed