An unaccounted for N2O sink in the surface water of the eastern subtropical South Pacific: Physical versus biological mechanisms

Nitrous oxide (N2O) is a trace gas affecting atmospheric radiative forcing through its greenhouse effect in the troposphere and destroying the ozone in the stratosphere. The oceans account for one-third of the global atmospheric N2O emissions, in which they are primarily cycled by nitrification and denitrification, with high N2O production in the subsurface waters. The surface waters are generally reported to be in equilibrium or slightly supersaturated with respect to the atmosphere. However, surface N2O sub-saturations have been observed in several regions of the world's oceans, such as off south-central Chile, which is bathed by the Sub-Antarctic Water Mass (SAAW), where N2O subsaturations as low as 35% have been registered during the austral spring and summer. An analysis of the mechanisms driving such surface N2O subsaturations (physical or biological) showed that physical mechanisms were not responsible for the high surface N2O deficit. In contrast, in situ potential experiments in surface waters with 151120 addition showed an active biological N2O fixation (between 0.43 and 87.34 nmol/L/d), with the highest N2O fixation rates associated with the SAAW (25.25-25.75 kg/m(3)). Additionally, incubation experiments with (N2O)-N-15 in surface water samples from one oceanic station showed high N-15-POM enrichment (0.44 parts per thousand) and an inhibition of N-15-POM enrichment when an additional nitrogen source was added (NO2- and NH4+). These results suggest the existence of a mechanism able to use several nitrogen sources, including N2O. Molecular analyses (16S rRNA gene) from these experiments showed the presence of three major groups of bacteria: Gammaproteobacteria, Flavobacteria and Cyanobacteria, with Synechococcus sp. being the dominant group in the culture. However, the analysis of the nifH gene showed a taxonomic affiliation to the order Stigonematales associated with Mastigocladus sp. and Fischerella sp. and the order Oscillatoriales associated with Trichodesmium sp. Finally, the oceanic region exhibiting surface N2O subsaturations acts as a sink for atmospheric N2O, consuming similar to 11.4 Gg of N2O in a six-month period. The N2O levels in the sink are 75% higher than those of the reported N2O source from the coastal band. The balance between the oceanic region and the coastal band results in a sink region of 4.94 Gg of N2O during this period. (C) 2014 Elsevier Ltd. All rights reserved.