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

NITROUS oxide plays an important part in atmospheric chemistry; it is a 'greenhouse' gas and has the potential to destroy ozone. Concentrations of N 2 O in surface waters of the oceans are generally close to equilibrium with the atmosphere, but sub-surface sea water is supersaturated with N 2 O 1–5 . The oceans are now regarded as a source 4 for atmospheric N 2 O by production through the oxidation of ammonium ('nitrification') 1,3 , except in extremely oxygen-deficient waters 6 . Nevertheless, the possibility that N 2 O might be produced through the reduction of nitrate ('denitrification') cannot be ruled out 5,7 . The distribution of 15 N in atmospheric and oceanic N 2 O can be used to investigate the behaviour of N 2 O in the oceans 8,9 . Here we report simultaneous measurements of the nitrification rates and nitrogen isotope data for N 2 O and NO 3 – produced in the western North Pacific Ocean. The amount of N 2 O produced by nitrification is much lower than that expected from the production of NO 3 – . The N 2 O in the oxygen-deficient layer is more enriched in 15 N than NO 3 – . These results imply that the contribution of nitrification to the production of N 2 O is lower than previously thought and that denitrification is primarily responsible for the production of N 2 O.