Abstract. The mechanisms of microbial nitrous oxide (N 2 O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N 2 O sources in natural environments. This study compares hydrographic, N 2 O concentration, and N 2 O isotopic and isotopomeric data from three stations along a coast-perpendicular transect in the South Pacific Ocean, extending from the center (Sts. GYR and EGY) of the subtropical oligotrophic gyre (∼26 • S; 114 • W) to the upwelling zone (St. UPX) off the central Chilean coast (∼34 • S). Although AOU/N 2 O and NO − 3 trends support the idea that most of the N 2 O (mainly from intermediate water (200-600 m)) comes from nitrification, N 2 O isotopomeric composition (intramolecular distribution of 15 N isotopes) expressed as SP (site preference of 15 N) shows low values (10 to 12‰) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N 2 O mediated by ammonium oxidizers). The coincidence of this SP signal with high -stability layer, where sinking organic particles can accumulate, suggests that N 2 O could be produced by nitrifier denitrification inside particles. It is postulated that deceleration of particles in the pycnocline can modify the advection -diffusion balance inside particles, allowing the accumulation of nitrite and O 2 depletion suitable for nitrifier denitrication. As lateral advection seems to be relatively insignificant in the gyre, in situ nitrifier denitrification could account for 40-50% of the N 2 O produced in this Correspondence to: J. Charpentier (jcharpentier@profc.udec.cl) layer. In contrast, coastal upwelling system is characterized by O 2 deficient condition and some N deficit in a eutrophic system. Here, N 2 O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N 2 O production processes, which presumably reflect both the effect of nitrification and denitrification at low O 2 levels on N 2 O production, but net N 2 O consumption by denitrification was not observed.