Bulk uptake 15N2: Difference between revisions

Latest revision as of 16:28, 11 May 2026

Data types › Nitrogen Fixation › Bulk uptake 15N2

N₂ fixation (bulk ¹⁵N₂)
Approach: ¹⁵N₂ stable isotope tracer uptake
Context: incubation, in situ
Spatial scale: point sample
Temporal scale: hours to one day
Units: nmol N L^-1 d^-1; µmol N m^-2 d^-1 (depth-integrated)
Community captured: bulk, size-fractionated
Co-measurements: background δ¹⁵N (PON), ¹⁵N₂ enrichment of dissolved phase; can couple with ¹³CO₂ uptake

Method Overview

¹⁵N₂ gas is dissolved into seawater and added to incubation bottles, labelling the dissolved N₂ pool. Diazotrophic microorganisms fix the labelled N₂, incorporating ¹⁵N into their biomass. After incubation, the particulate fraction is collected on pre-combusted GF/F filters; the ¹⁵N enrichment in particulate nitrogen (PN) is measured by EA-IRMS. The N₂ fixation rate is calculated from the ¹⁵N excess in PN relative to the ¹⁵N enrichment of the dissolved N₂ pool, measured by MIMS or GC-IRMS^[1].

Two labelling approaches are used: (1) direct injection of ¹⁵N₂ gas bubbles followed by vigorous shaking and equilibration; and (2) the dissolution method, in which pre-dissolved ¹⁵N₂-enriched water is injected, achieving more homogeneous labelling. The method can be coupled with ¹³CO₂ uptake in the same incubation to simultaneously measure N₂ fixation and carbon fixation by diazotrophs.

Scale of measurement

Each incubation provides a point measurement. Depth-integrated fixation rates are obtained by incubating bottles collected from multiple depths and integrating over the euphotic zone. Incubation duration is typically hours to one day.

Data generated

N₂ fixation rates in nmol N L^-1 d^-1 or µmol N m^-2 d^-1 (depth-integrated). Rates can be size-fractionated (e.g., 0.2–10 µm versus > 10 µm) to attribute fixation to different diazotroph size classes (unicellular vs. filamentous). Fixed nitrogen can also be tracked into the dissolved organic nitrogen (DON) pool.

Units & currency

Units are nmol N L^-1 d^-1 or µmol N m^-2 d^-1. The currency is nitrogen.

Sample size

Typical samples are 1–5 L in volume per incubation.

Repositories & databases

Limitations

The ¹⁵N₂ enrichment of the dissolved pool must be measured accurately; under-estimation of the ¹⁵N₂ enrichment (e.g., due to incomplete equilibration of bubbles) leads to overestimation of fixation rates. Bubble injection produces heterogeneous enrichment across replicate bottles; the dissolution method mitigates this. Diazotroph community composition affects the timing of fixation: Crocosphaera fixes N₂ primarily at night, UCYN-A primarily during the day. Bottle effects apply.

Example Applications & Protocols

Classic examples

Montoya et al. (1996) A simple, high-precision, high-sensitivity tracer assay for N₂ fixation ^[1]
Wilson et al. (2012) Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean ^[2]
Großkopf et al. (2012) Doubling of marine dinitrogen-fixation rates based on direct measurements ^[3]

Recent applications

Tschitschko et al. (2024) Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean ^[4]

Common calculations/conversions

N₂ fixation rate (nmol N L^-1 d^-1) = [(¹⁵N_PN,final − ¹⁵N_PN,initial) / (¹⁵N_N2,dissolved − ¹⁵N_natural)] × [PN] / incubation time.
Depth-integrated rate (µmol N m^-2 d^-1) = ∫ rate(z) dz over the euphotic zone.

References

↑ ^1.0 ^1.1 Montoya, J. P., Voss, M., Kähler, P., & Capone, D. G. (1996). A simple, high-precision, high-sensitivity tracer assay for N₂ fixation. Applied and Environmental Microbiology, 62(3), 986–993. https://doi.org/10.1128/aem.62.3.986-993.1996
↑ Wilson, S. T., Böttjer, D., Church, M. J., & Karl, D. M. (2012). Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean. Applied and Environmental Microbiology, 78(18), 6516–6523. https://doi.org/10.1128/AEM.01146-12
↑ Großkopf, T., Mohr, W., Baustian, T., Schunck, H., Gill, D., Kuypers, M. M. M., Lavik, G., Schmitz, R. A., Wallace, D. W. R., & LaRoche, J. (2012). Doubling of marine dinitrogen-fixation rates based on direct measurements. Nature, 488(7411), 361–364. https://doi.org/10.1038/nature11338
↑ Tschitschko, B., Landa, M., Cheung, S., Zheng, H., Williams, C. M., Vergin, K., Giovannoni, S. J., Rocap, G., & Lindell, D. (2024). Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean. Nature, 630, 899–904. https://doi.org/10.1038/s41586-024-07495-w

[Montoya1996-1] 1.0 ^1.1 Montoya, J. P., Voss, M., Kähler, P., & Capone, D. G. (1996). A simple, high-precision, high-sensitivity tracer assay for N₂ fixation. Applied and Environmental Microbiology, 62(3), 986–993. https://doi.org/10.1128/aem.62.3.986-993.1996

[Wilson2012-2] Wilson, S. T., Böttjer, D., Church, M. J., & Karl, D. M. (2012). Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean. Applied and Environmental Microbiology, 78(18), 6516–6523. https://doi.org/10.1128/AEM.01146-12

[Großkopf2012-3] Großkopf, T., Mohr, W., Baustian, T., Schunck, H., Gill, D., Kuypers, M. M. M., Lavik, G., Schmitz, R. A., Wallace, D. W. R., & LaRoche, J. (2012). Doubling of marine dinitrogen-fixation rates based on direct measurements. Nature, 488(7411), 361–364. https://doi.org/10.1038/nature11338

[Tschitschko2024-4] Tschitschko, B., Landa, M., Cheung, S., Zheng, H., Williams, C. M., Vergin, K., Giovannoni, S. J., Rocap, G., & Lindell, D. (2024). Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean. Nature, 630, 899–904. https://doi.org/10.1038/s41586-024-07495-w

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@@ Line 1: / Line 1: @@
 {{DISPLAYTITLE:Bulk uptake <sup>15</sup>N<sub>2</sub>}}
+{{BreadcrumbsNFixation}}
+{{BreadcrumbsPrimaryProduction}}
+* [[Page authors|Page authors]]: [[PRIMO]]
+* [[Responsible curator|Responsible curator]]:  [[User:Hagi BucknWise|Hagen Buck-Wiese]]
+----
+__TOC__
+<div class="model-box">
+{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;"
+! N<sub>2</sub> fixation (bulk <sup>15</sup>N<sub>2</sub>)
+|-
+| '''Approach:''' <sup>15</sup>N<sub>2</sub> stable isotope tracer uptake
+|-
+| '''Context:''' incubation, ''in situ''
+|-
+| '''Spatial scale:''' point sample
+|-
+| '''Temporal scale:''' hours to one day
+|-
+| '''Units:''' nmol N L<sup>-1</sup> d<sup>-1</sup>; µmol N m<sup>-2</sup> d<sup>-1</sup> (depth-integrated)
+|-
+| '''Community captured:''' bulk, size-fractionated
+|-
+| '''Co-measurements:''' background δ<sup>15</sup>N (PON), <sup>15</sup>N<sub>2</sub> enrichment of dissolved phase; can couple with <sup>13</sup>CO<sub>2</sub> uptake
+|}
+</div>
+<div style="clear:both"></div>
+== Method Overview ==
+<sup>15</sup>N<sub>2</sub> gas is dissolved into seawater and added to incubation bottles, labelling the dissolved N<sub>2</sub> pool. Diazotrophic microorganisms fix the labelled N<sub>2</sub>, incorporating <sup>15</sup>N into their biomass. After incubation, the particulate fraction is collected on pre-combusted GF/F filters; the <sup>15</sup>N enrichment in particulate nitrogen (PN) is measured by EA-IRMS. The N<sub>2</sub> fixation rate is calculated from the <sup>15</sup>N excess in PN relative to the <sup>15</sup>N enrichment of the dissolved N<sub>2</sub> pool, measured by MIMS or GC-IRMS<ref name="Montoya1996">Montoya, J. P., Voss, M., Kähler, P., & Capone, D. G. (1996). A simple, high-precision, high-sensitivity tracer assay for N<sub>2</sub> fixation. ''Applied and Environmental Microbiology'', 62(3), 986–993. https://doi.org/10.1128/aem.62.3.986-993.1996</ref>.
+Two labelling approaches are used: (1) direct injection of <sup>15</sup>N<sub>2</sub> gas bubbles followed by vigorous shaking and equilibration; and (2) the dissolution method, in which pre-dissolved <sup>15</sup>N<sub>2</sub>-enriched water is injected, achieving more homogeneous labelling. The method can be coupled with <sup>13</sup>CO<sub>2</sub> uptake in the same incubation to simultaneously measure N<sub>2</sub> fixation and carbon fixation by diazotrophs.
+=== Scale of measurement ===
+Each incubation provides a point measurement. Depth-integrated fixation rates are obtained by incubating bottles collected from multiple depths and integrating over the euphotic zone. Incubation duration is typically hours to one day.
+=== Data generated ===
+N<sub>2</sub> fixation rates in nmol N L<sup>-1</sup> d<sup>-1</sup> or µmol N m<sup>-2</sup> d<sup>-1</sup> (depth-integrated). Rates can be size-fractionated (e.g., 0.2–10 µm versus > 10 µm) to attribute fixation to different diazotroph size classes (unicellular vs. filamentous). Fixed nitrogen can also be tracked into the dissolved organic nitrogen (DON) pool.
+=== Units & currency ===
+Units are nmol N L<sup>-1</sup> d<sup>-1</sup> or µmol N m<sup>-2</sup> d<sup>-1</sup>. The currency is nitrogen.
+=== Sample size ===
+Typical samples are 1–5 L in volume per incubation.
+=== Repositories & databases ===
+== Limitations ==
+The <sup>15</sup>N<sub>2</sub> enrichment of the dissolved pool must be measured accurately; under-estimation of the <sup>15</sup>N<sub>2</sub> enrichment (e.g., due to incomplete equilibration of bubbles) leads to overestimation of fixation rates. Bubble injection produces heterogeneous enrichment across replicate bottles; the dissolution method mitigates this. Diazotroph community composition affects the timing of fixation: Crocosphaera fixes N<sub>2</sub> primarily at night, UCYN-A primarily during the day. Bottle effects apply.
+== Example Applications & Protocols ==
+=== Classic examples ===
+* Montoya et al. (1996) ''A simple, high-precision, high-sensitivity tracer assay for N<sub>2</sub> fixation'' <ref name="Montoya1996" />
+* Wilson et al. (2012) ''Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean'' <ref name="Wilson2012">Wilson, S. T., Böttjer, D., Church, M. J., & Karl, D. M. (2012). Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean. ''Applied and Environmental Microbiology'', 78(18), 6516–6523. https://doi.org/10.1128/AEM.01146-12</ref>
+* Großkopf et al. (2012) ''Doubling of marine dinitrogen-fixation rates based on direct measurements'' <ref name="Großkopf2012">Großkopf, T., Mohr, W., Baustian, T., Schunck, H., Gill, D., Kuypers, M. M. M., Lavik, G., Schmitz, R. A., Wallace, D. W. R., & LaRoche, J. (2012). Doubling of marine dinitrogen-fixation rates based on direct measurements. ''Nature'', 488(7411), 361–364. https://doi.org/10.1038/nature11338</ref>
+=== Recent applications ===
+* Tschitschko et al. (2024) ''Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean'' <ref name="Tschitschko2024">Tschitschko, B., Landa, M., Cheung, S., Zheng, H., Williams, C. M., Vergin, K., Giovannoni, S. J., Rocap, G., & Lindell, D. (2024). Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean. ''Nature'', 630, 899–904. https://doi.org/10.1038/s41586-024-07495-w</ref>
+=== Common calculations/conversions ===
+* N<sub>2</sub> fixation rate (nmol N L<sup>-1</sup> d<sup>-1</sup>) = [(<sup>15</sup>N<sub>PN,final</sub> − <sup>15</sup>N<sub>PN,initial</sub>) / (<sup>15</sup>N<sub>N2,dissolved</sub> − <sup>15</sup>N<sub>natural</sub>)] × [PN] / incubation time.
+* Depth-integrated rate (µmol N m<sup>-2</sup> d<sup>-1</sup>) = ∫ rate(z) dz over the euphotic zone.
+== References ==
+[[Category:Main Pages|Model types]]