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Fernández-Castro, B. . et. al.-- 10 pages, 5 figures, 2 tables, supplementary information http://www.nature.com/ncomms/2015/150909/ncomms9002/full/ncomms9002.html#supplementary-information ; he input of new nitrogen into the euphotic zone constrains the export of organic carbon to the deep ocean and thereby the biologically mediated long-term CO2 exchange between the ocean and atmosphere. In low-latitude open-ocean regions, turbulence-driven nitrate diffusion from the ocean's interior and biological fixation of atmospheric N2 are the main sources of new nitrogen for phytoplankton productivity. With measurements across the tropical and subtropical Atlantic, Pacific and Indian oceans, we show that nitrate diffusion (171±190 μmol m−2 d−1) dominates over N2 fixation (9.0±9.4 μmol m−2 d−1) at the time of sampling. Nitrate diffusion mediated by salt fingers is responsible for ca. 20% of the new nitrogen supply in several provinces of the Atlantic and Indian Oceans. Our results indicate that salt finger diffusion should be considered in present and future ocean nitrogen budgets, as it could supply globally 0.23–1.00 Tmol N yr−1 to the euphotic zone. © 2015 Macmillan Publishers Limited. All rights reserved ; Funding for this study was provided by the Spanish Ministry of Science and Innovation under the research projects CSD2008-00077 to C.M. Duarte and CTM2012-30680 to B. Mouriño-Carballido. [.] B. Fernández-Castro thanks the Spanish government for a FPU grant (AP2010-5594) ; Peer Reviewed