Global oceanic production of nitrous oxide

Abstract

We use transient time distributions calculated from tracer data together with in situ measurements of nitrous oxide (N 2 O) to estimate the concentration of biologically produced N 2 O and N 2 O production rates in the ocean on a global scale. Our approach to estimate the N 2 O production rates integrates the effects of potentially varying production and decomposition mechanisms along the transport path of a water mass. We estimate that the oceanic N 2 O production is dominated by nitrification with a contribution of only approximately 7 per cent by denitrification. This indicates that previously used approaches have overestimated the contribution by denitrification. Shelf areas may account for only a negligible fraction of the global production; however, estuarine sources and coastal upwelling of N 2 O are not taken into account in our study. The largest amount of subsurface N 2 O is produced in the upper 500 m of the water column. The estimated global annual subsurface N 2 O production ranges from 3.1 ± 0.9 to 3.4 ± 0.9 Tg N yr −1 . This is in agreement with estimates of the global N 2 O emissions to the atmosphere and indicates that a N 2 O source in the mixed layer is unlikely. The potential future development of the oceanic N 2 O source in view of the ongoing changes of the ocean environment (deoxygenation, warming, eutrophication and acidification) is discussed.