An unaccounted for N 2 O sink in the surface water of the eastern subtropical South Pacific: Physical versus biological mechanisms q Marcela Cornejo a,⇑ , Alejandro A. Murillo b , Laura Farías c,d a Escuela de Ciencias del Mar, Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile b Instituto Milenio de Oceanografía (IMO-Chile), Universidad de Concepción, P.O. Box 1313 Concepción 3, Concepción, Chile c Laboratorio de Oceanografía Física y Clima, P.O. Box 160-C, Concepción, Chile d Departamento de Oceanografía y Centro de Cambio Climático y Resiliencia, P.O. Box 160-C, Concepción, Chile article info Article history: Received 15 February 2014 Received in revised form 12 December 2014 Accepted 22 December 2014 Available online 8 January 2015 abstract Nitrous oxide (N 2 O) 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 N 2 O emissions, in which they are primarily cycled by nitrification and denitrification, with high N 2 O 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 N 2 O sub-satu- rations 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 N 2 O subsaturations as low as 35% have been registered during the austral spring and summer. An analysis of the mechanisms driving such surface N 2 O subsaturations (physical or biological) showed that physical mechanisms were not responsible for the high surface N 2 O deficit. In contrast, in situ potential experiments in surface waters with 15 N 2 O addition showed an active biological N 2 O fixation (between 0.43 and 87.34 nmol/L/d), with the highest N 2 O fixa- tion rates associated with the SAAW (25.25–25.75 kg/m 3 ). Additionally, incubation experiments with 15 N 2 O in surface water samples from one oceanic station showed high 15 N-POM enrichment (0.44‰) and an inhibition of 15 N-POM enrichment when an additional nitrogen source was added (NO 2  and NH 4 + ). These results suggest the existence of a mechanism able to use several nitrogen sources, including N 2 O. Molecular analyses (16S rRNA gene) from these experiments showed the presence of three major groups of bacteria: Gammaproteobacteria, Flavobacteria and Cyano- bacteria, 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 N 2 O subsaturations acts as a sink for atmospheric N 2 O, consuming 11.4 Gg of N 2 O in a six-month period. The N 2 O levels in the sink are 75% higher than those of the reported N 2 O 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 N 2 O during this period. Ó 2014 Elsevier Ltd. All rights reserved. Introduction The global ocean is one of the primary natural sources of atmo- spheric N 2 O (4–16 Tg N per year, Bange et al., 2010), a powerful greenhouse and ozone-lytic gas (Prather et al., 2001). The oceanic contribution of N 2 O to the atmosphere is far from being homogeneous and presents a high spatial variability, responding to regional oceanographic conditions (e.g., upwelling, cooling, hyp- oxic–anoxic events, productivity, etc.). N 2 O cycling occurs primar- ily in subsurface waters, where oxygen concentrations decrease and two microbial pathways, denitrification and nitrification, can take place, either separately or coupled. Accordingly, N 2 O can reach the surface by diffusion and/or vertical transport through upwelling. In the eastern boundaries of the oceans, the upwelling processes are linked to oxygen minimum zones, the main zones of N 2 O accumulation and an exchange to the atmosphere (Naqvi et al., 2005; Cornejo et al., 2007). These regions represent a small fraction of the oceans, whereas, indeed, 80% of the surface waters globally appear to be close to the N 2 O equilibrium with the atmo- http://dx.doi.org/10.1016/j.pocean.2014.12.016 0079-6611/Ó 2014 Elsevier Ltd. All rights reserved. q Key Points: Surface N 2 O subsaturations in the oceanic surface layer of the ESP off central Chile were characterized, evaluated using hydrographic, wind data. The oceanic region was identified to be an important sink of atmospheric N 2 O, whose main driving process is biological fixation rather than physical factors. ⇑ Corresponding author. E-mail address: marcela.cornejo@ucv.cl (M. Cornejo). Progress in Oceanography 137 (2015) 12–23 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean