Mechanisms of nitrous oxide production in the subtropical North Pacific based on determinations of the isotopic abundances of nitrous oxide and di-oxygen

Abstract

In this study, we compare stable isotopic compositions of di-oxygen (O2) and nitrous oxide (N2O) in two depth profiles at the well-characterized deep water station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the subtropical North Pacific gyre to attain an understanding of the mechanisms of N2O production. The δ18O of O2 varied from values indicative of an atmospheric origin near the surface (24.7‰), to minimum values reflective of a predominance of photosynthesis over respiration between the surface and 200 m (as low as 22.2‰), to maximum values as high as 36.6‰ in association with the O2 minimum near 800 m. A similar pattern of isotopic variation was evident in the δ18O of N2O, however, values were enriched by approximately 20‰. The similar pattern of variation in δ18O with depth is consistent with an origin of O in N2O from dissolved O2 via the nitrification of intermediate compounds NH2OH or NO. Between the depths of 350 and 500 m, however, the distinction in the isotopic composition of N2O and O2 was reduced to as little as 12‰. This decrease in the difference between the δ18O of N2O and that of O2 with depth indicates either a reduction in the magnitude of isotopic discrimination during nitrification or a contribution of O in N2O from water via the reduction of NO2−. Two mechanisms of N2O production via nitrification may, therefore, occur in the subtropical Pacific; release from the nitrification of NH2OH or NO at most depths and reduction of NO2− between 350 and 500 m. In that, the carbon flux decreases markedly over a similar depth interval at this locale (Karl, D.M., Knauer, G.A., Martin, J.H., 1988. Downward flux of particulate organic matter in the ocean: A particle decomposition paradox. Nature 332, 438–441), this distinct mechanism of N2O production between 350 and 500 m may be associated with the mineralization of organic matter from sinking particles. Low O2 or anoxic micro-environments within particles within this depth interval may be maintained by lower ambient O2 than at the surface and high rates of microbial activity supported by the mineralization of organic matter. Such conditions may facilitate an environment conducive to N2O production via NO2− reduction.