Abstract

The mechanisms of microbial nitrous oxide (N 2O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N 2O sources in natural environments. This study compares hydrographic, N 2O concentration, and N 2O 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 AOUZ/N 2O and NO 3 - trends support the idea that most of the N 2O (mainly from intermediate water (200-600 m)) comes from nitrification, N 2O isotopomeric composition (intramolecular distribution of 15N isotopes) expressed as SP (site preference of 15N) shows low values (10 to 12‰) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N 2O mediated by ammonium oxidizers). The coincidence of this SP signal with high - stability layer, where sinking organic particles can accumulate, suggests that N 2O 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 2O produced in this layer. In contrast, coastal upwelling system is characterized by O 2 deficient condition and some N deficit in a eutrophic system. Here, N 2O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N 2O production processes, which presumably reflect both the effect of nitrification and denitrification at low O 2 levels on N 2O production, but net N 2O consumption by denitrification was not observed.