Natural Abundances of 15N and 18O

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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Denitrification isotope fingerprint (δ15N, δ18O-NO3)
Approach: natural abundance IRMS of δ15N and δ18O in nitrate
Context: in situ
Spatial scale: point sample
Temporal scale: days to weeks (integrative)
Units: ‰ (per mil) δ15N-NO3- and δ18O-NO3-
Community captured: bulk
Co-measurements: nitrate concentrations

Method Overview

The dual isotopic composition of dissolved nitrate — δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- — is measured in water samples collected in situ. During denitrification, both ¹⁴N and ¹⁶O are preferentially consumed, enriching the residual nitrate pool in both ¹⁵N and ¹⁸O in a near-1:1 molar ratio. Nitrification, by contrast, produces new NO₃^- with low δ¹⁵N and δ¹⁸O, diluting isotopic signatures. The divergence or convergence of δ¹⁵N and δ¹⁸O 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 NO₃^- to N₂O 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

δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- values (‰ vs. air N₂ and VSMOW). The slope of the δ¹⁵N vs. δ¹⁸O 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).

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 δ¹⁸O-NO₃^- and ambient water can complicate the δ¹⁸O 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

Recent applications

• Bourbonnais et al. (2024) Advances in understanding the marine nitrogen cycle in the GEOTRACES era

Common calculations/conversions

• Isotopic enrichment factor ε (‰) = 1000 × (α − 1), where α = k_light/k_heavy; for denitrification ε_15N ≈ −15 to −30‰.