Nitrous oxide (N2O), a trace gas whose concentration is increasing in
the atmosphere, plays an important role in both radiative forcing and
stratospheric ozone depletion(1,2). Its biogeochemical cycle has thus
come under intense scrutiny in recent years. Despite these efforts, th
e global budget of N2O remains unresolved, and the nature and magnitud
e of the sources and sinks continue to be debated(3-5) despite the con
straints that can be provided by characterizations of the gas(6,7). We
report here the results of dual-isotope measurements of N2O from the
water column of the subtropical North Pacific Ocean. Nitrous oxide wit
hin the lower-euphotic and upper-aphotic zones is depleted in both N-1
5 and O-18 relative to its tropospheric and deep-ocean composition. Th
ese findings are consistent with a prediction, based on global mass-ba
lance considerations, of a near-surface isotopically depleted oceanic
N2O source(4). Our results indicate that this source, probably produce
d by bacterial nitrification, contributes significantly to the ocean-a
tmosphere flux of N2O in the oligotrophic subtropical North Pacific Oc
ean. This source may act to buffer the isotopic composition of troposp
heric N2O and is quantitatively significant in the global tropospheric
N2O budget. Because dissolved gases in near-surface waters are more r
eadily exchanged with the atmospheric reservoir than those in deep wat
ers, the existence of a quantitatively significant N2O source at a rel
atively shallow depth has potentially important implications for the s
usceptibility of the source, and the ocean-atmosphere flux, to climati
c influences.