the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Insights into nitrogen fixation below the euphotic zone: trials in an oligotrophic marginal sea and global compilation
Abstract. Nitrogen (N2) fixation, the energetically expensive conversion of N2 to ammonia, plays an important role in balancing the global nitrogen budget. Defying historic paradigms, recent studies have detected non-cyanobacterial N2 fixation in deep, dark oceanic waters. Even low volumetric rates can be significant considering the large volume of these waters. However, measuring aphotic N2 fixation is an analytical challenge due to the low particulate nitrogen (PN) concentrations. Here, we investigated N2 fixation rates in aphotic waters in the South China Sea (SCS). To increase the sensitivity of N2 fixation rate measurements, we applied a novel approach requiring only 0.28 μg N for measuring the isotopic composition of particulate nitrogen. We conducted parallel 15N2-enriched incubations in ambient seawater, seawater amended with amino acids and poisoned (HgCl2) controls, along with incubations that received no tracer additions to distinguish biological N2 fixation. Experimental treatments differed significantly from our two types of controls, those receiving no additions and killed controls. Amino acid additions masked N2 fixation signals due to the uptake of added 14N-amino acid. Results show that the maximum dark N2 fixation rates (1.28 ± 0.85 nmol N L−1 d−1) occurred within upper 200 m, while rates below 200 m were mostly lower than 0.1 nmol N L−1 d−1. Nevertheless, N2 fixation rates between 200 and 1000 m accounted for 39 ± 32 % of depth-integrated dark N2 fixation rates in the upper 1000 m, which is comparable to the areal nitrogen inputs via atmospheric deposition. Globally, we found that aphotic N2 fixation studies conducted in oxygenated environments yielded rates similar to those from the SCS (< 1 nmol N L−1 d−1), regardless of methods, while higher rates were occasionally observed in low-oxygen (< 62 µM) regions. Regression analysis suggests that particulate nitrogen concentrations could be a predictive proxy for detectable aphotic N2 fixation in the SCS and eastern tropical south Pacific. Our results provide the first insight into aphotic N2 fixation in SCS and support the importance of the aphotic zone as a globally-important source of new nitrogen to the ocean.
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Interactive discussion
Status: closed
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RC1: 'Comment on bg-2021-104', Yubin Raut, 31 Oct 2021
See attached word document for list of recommendations to improve the manuscript.
- AC1: 'Reply on RC1', Shuh-Ji Kao, 16 Feb 2022
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EC1: 'Comment on bg-2021-104', Markus Kienast, 14 Jan 2022
Dear authors,
Despite my best efforts, I have not been able to secure a second referee report for your contribution, unfortunately. In order to move your manuscript along, we will close the discussion with only one report now. I would like to apologize for the most unfortunate delay.
Please upload your response to the first referee report at your earliest convenience.
Sincerely,
Markus
Citation: https://doi.org/10.5194/bg-2021-104-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on bg-2021-104', Yubin Raut, 31 Oct 2021
See attached word document for list of recommendations to improve the manuscript.
- AC1: 'Reply on RC1', Shuh-Ji Kao, 16 Feb 2022
-
EC1: 'Comment on bg-2021-104', Markus Kienast, 14 Jan 2022
Dear authors,
Despite my best efforts, I have not been able to secure a second referee report for your contribution, unfortunately. In order to move your manuscript along, we will close the discussion with only one report now. I would like to apologize for the most unfortunate delay.
Please upload your response to the first referee report at your earliest convenience.
Sincerely,
Markus
Citation: https://doi.org/10.5194/bg-2021-104-EC1
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Cited
Siqi Wu
Moge Du
Xianhui Sean Wan
Corday Selden
Mar Benavides
Sophie Bonnet
Robert Hamersley
Carolin R. Löscher
Margaret R. Mulholland
Xiuli Yan
Shuh-Ji Kao
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