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

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 ($\text{24.7‰}$), to minimum values reflective of a predominance of photosynthesis over respiration between the surface and 200 m (as low as 22.2$\text{‰}$), to maximum values as high as $\text{36.6‰}$ 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 $\text{20‰}$. 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 $\text{12‰}$. 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.