Abstract
Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes. The distribution, controlling factors, and flux of N2 fixation in the global ocean remain uncertain, partly because of the lack of methodological uniformity. The 15N2 tracer assay (the original bubble method → the 15N2-enriched seawater method → the modified bubble method) is the mainstream method for field measurements of N2 fixation rates (NFRs), among which the original bubble method is the most frequently used. However, accumulating evidence has suggested an underestimation of NFRs when using this method. To improve the availability of previous data, we compared NFRs measured by three 15N2 tracer assays in the South China Sea. Our results indicate that the relationship between NFRs measured by the original bubble method and the 15N2-enriched seawater method varies obviously with area and season, which may be influenced by incubation time, diazotrophic composition, and environmental factors. In comparison, the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable, indicating that the N2 fixation rates based on the original bubble methods may be underestimated by approximately 50%. Based on this result, we revised the flux of N2 fixation in the South China Sea to 40 mmol/(m2·a). Our results improve the availability and comparability of literature NFR data in the South China Sea. The comparison of the 15N2 tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N2 fixation in the marine nitrogen cycle.
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Acknowledgements
We are grateful to the project team members for sam** assistance. Thanks are also due to scientists who kindly shared physical, chemical and biological data. Samples were collected during the open research cruises NORC2013-07, NORC2014-07, NORC2017-05, NORC2017-06 and NORC2018-05, supported by Natural Science Foundation of China Shiptime Sharing Project.
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The National Natural Science Foundation of China under contract Nos 42076042 and 41721005; the Fund of Ministry of Science and Technology of China under contract No. 2017FY201403; the Fund of China Ocean Mineral Resources R&D Association under contract No. DY135-13-E2-03.
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Li, D., Zheng, M., Qiu, Y. et al. Comparative assessment of nitrogen fixation rate by 15N2 tracer assays in the South China Sea. Acta Oceanol. Sin. 42, 75–82 (2023). https://doi.org/10.1007/s13131-022-2092-3
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DOI: https://doi.org/10.1007/s13131-022-2092-3