Nitrification rates and 15N abundances of N2O and NO3− in the western North Pacific

Abstract

NITROUS oxide plays an important part in atmospheric chemistry; it is a 'greenhouse' gas and has the potential to destroy ozone. Concentrations of N2O in surface waters of the oceans are generally close to equilibrium with the atmosphere, but sub-surface sea water is supersaturated with N2O1–5. The oceans are now regarded as a source4 for atmospheric N2O by production through the oxidation of ammonium ('nitrification')1,3, except in extremely oxygen-deficient waters6. Nevertheless, the possibility that N2O might be produced through the reduction of nitrate ('denitrification') cannot be ruled out5,7. The distribution of 15N in atmospheric and oceanic N2O can be used to investigate the behaviour of N2O in the oceans8,9. Here we report simultaneous measurements of the nitrification rates and nitrogen isotope data for N2O and NO3– produced in the western North Pacific Ocean. The amount of N2O produced by nitrification is much lower than that expected from the production of NO3–. The N2O in the oxygen-deficient layer is more enriched in 15N than NO3–. These results imply that the contribution of nitrification to the production of N2O is lower than previously thought and that denitrification is primarily responsible for the production of N2O.