Comparative assessment of nitrogen fixation rate by 15N2 tracer assays in the South China Sea
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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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Key words:
- N2 fixation rate /
- South China Sea /
- 15N2 tracer assay
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Figure 1. Sampling locations in the South China Sea (SCS): a. stations in the open areas of the SCS from 2013 to 2017 (“A-”, “K-”, “C-”, “G-” and “D-” indicate cruises in August 2013, July 2014, March 2017, July 2017 and June 2018, “□” indicates the sampling stations of nifH in March 2017); b. stations in the Daya Bay (prefixed with “E-”, July 2015, December 2015, April 2016 and January 2018).
Figure 2. The range of NFRs in the open area of the SCS (the top margins and the bottom margins of the boxes represent the third quartile (Q3) and the first quartile (Q1) respectively. The upper limit and the lower limit were calculated as follows: upper limit = Q3 + (Q3 − Q1) × 1.5, lower limit = Q1 − (Q3 − Q1) × 1.5. Values larger than the upper limit or less than the lower limit were defined as outliers. The solid lines in the boxes represent the medians, and the dotted lines represent the average values).
Figure 5. The relationship between N2 fixation rates (NFRs) measured by the original bubble (B) method and the 15N2-enriched seawater (S) methods: the unit of NFRs observed in this study and in Großkopf et al. (2012) is nmol/(L·d) , and the unit of NFRs (depth-integrated NFRs) in Wilson et al. (2012) is µmol/(m2·d). To improve the visibility of Fig. 5, the unit of NFRs in the Daya Bay (DB) are converted to nmol/(L·d).
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