View source for Natural Abundances of 15N and 18O

{{BreadcrumbsNutrients}}

* [[Page authors|Page authors]]: [[PRIMO]]
* [[Responsible curator|Responsible curator]]:  [[User:Hagi BucknWise|Hagen Buck-Wiese]]
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! Denitrification isotope fingerprint (δ<sup>15</sup>N, δ<sup>18</sup>O-NO<sub>3</sub>)
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| '''Approach:''' natural abundance IRMS of δ<sup>15</sup>N and δ<sup>18</sup>O in nitrate
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| '''Context:''' ''in situ''
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| '''Spatial scale:''' point sample
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| '''Temporal scale:''' days to weeks (integrative)
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| '''Units:''' ‰ (per mil) δ<sup>15</sup>N-NO<sub>3</sub><sup>-</sup> and δ<sup>18</sup>O-NO<sub>3</sub><sup>-</sup>
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| '''Community captured:''' bulk
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| '''Co-measurements:''' nitrate concentrations
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== Method Overview ==

The dual isotopic composition of dissolved nitrate — δ<sup>15</sup>N-NO<sub>3</sub><sup>-</sup> and δ<sup>18</sup>O-NO<sub>3</sub><sup>-</sup> — is measured in water samples collected in situ. During denitrification, both <sup>14</sup>N and <sup>16</sup>O are preferentially consumed, enriching the residual nitrate pool in both <sup>15</sup>N and <sup>18</sup>O in a near-1:1 molar ratio. Nitrification, by contrast, produces new NO<sub>3</sub><sup>-</sup> with low δ<sup>15</sup>N and δ<sup>18</sup>O, diluting isotopic signatures. The divergence or convergence of δ<sup>15</sup>N and δ<sup>18</sup>O in nitrate therefore serves as a qualitative and semi-quantitative indicator of active denitrification versus nitrification. Isotope ratio mass spectrometry (IRMS) is performed after conversion of NO<sub>3</sub><sup>-</sup> to N<sub>2</sub>O by bacterial denitrifiers (denitrifier method).

=== Scale of measurement ===

Point sample. The isotopic signal integrates over the timescale at which the nitrate pool turns over through biological activity, typically days to weeks in the euphotic zone and much longer in the deep ocean.

=== Data generated ===

δ<sup>15</sup>N-NO<sub>3</sub><sup>-</sup> and δ<sup>18</sup>O-NO<sub>3</sub><sup>-</sup> values (‰ vs. air N<sub>2</sub> and VSMOW). The slope of the δ<sup>15</sup>N vs. δ<sup>18</sup>O relationship (ideally 1:1 for denitrification) and deviations from it constrain the relative roles of denitrification and nitrification.

=== Units & currency ===

Units are ‰ (per mil). The currency is nitrogen.

=== Sample size ===

Typical samples are 20–100 mL in volume (frozen immediately after collection for later analysis).

=== Repositories & databases ===

== Limitations ==

The isotopic signatures of denitrification can be driven by multiple simultaneous nitrate sources and sinks with overlapping signatures, making unique attribution difficult without additional data. Isotope exchange between δ<sup>18</sup>O-NO<sub>3</sub><sup>-</sup> and ambient water can complicate the δ<sup>18</sup>O signal. Quantitative interpretation requires isotope mass-balance or inverse models. The method cannot distinguish between different denitrifying organisms or environments (water column vs. sediment) from water column measurements alone.

== Example Applications & Protocols ==

=== Classic examples ===
*

=== Recent applications ===
* Bourbonnais et al. (2024) ''Advances in understanding the marine nitrogen cycle in the GEOTRACES era'' <ref name="Bourbonnais2024">Bourbonnais, A., Reistetter, E., Bange, H. W., Duhamel, S., Fripiat, F., Granger, J., Loick-Wilde, N., Marconi, D., Somes, C., Tuerena, R. E., & Altabet, M. A. (2024). Advances in understanding the marine nitrogen cycle in the GEOTRACES era. ''Oceanography'', 37(2). https://doi.org/10.5670/oceanog.2024.406</ref>

=== Common calculations/conversions ===
* Isotopic enrichment factor ε (‰) = 1000 × (α − 1), where α = k<sub>light</sub>/k<sub>heavy</sub>; for denitrification ε<sub>15N</sub> ≈ −15 to −30‰.

== References ==

[[Category:Main Pages|Model types]]