Natural abundance of N and O isotopes in nitrate, nitrite and ammonium

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

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N-cycle isotope signatures (natural abundance)
Approach: IRMS measurement of natural abundance δ15N and δ18O in DIN pools
Context: in situ
Spatial scale: point sample
Temporal scale: integrative (based on substrate turnover time)
Units: ‰ (per mil)
Community captured: bulk
Co-measurements: ambient [NH4+], [NO2-], [NO3-]

Method Overview

The natural abundance of stable nitrogen (¹⁵N/¹⁴N, expressed as δ¹⁵N) and oxygen (¹⁸O/¹⁶O, expressed as δ¹⁸O) isotopes in dissolved nitrate, nitrite, and ammonium is measured by IRMS. Each transformation in the nitrogen cycle (nitrification, denitrification, assimilation) imparts characteristic isotopic fractionation, enriching the residual substrate in the heavier isotope. The dual isotopic composition of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) is particularly informative because denitrification elevates both δ¹⁵N and δ¹⁸O in a near-1:1 ratio, while nitrification adds NO₃^- with relatively low δ¹⁵N and δ¹⁸O values, allowing the two processes to be distinguished.

Sample volumes of 30–60 mL are typically frozen for later IRMS analysis. Methods include the denitrifier method (conversion of NO₃^- to N₂O by denitrifying bacteria) and the azide method for NO₂^-.

Scale of measurement

Point sample; the isotopic signal integrates over the turnover timescale of the substrate, which can range from hours (NH₄⁺ in productive waters) to years (deep ocean NO₃^-). This makes the method complementary to direct rate measurements.

Data generated

δ¹⁵N and δ¹⁸O values (‰ vs. air N₂ and VSMOW respectively) for NO₃^-, NO₂^-, and/or NH₄⁺. Interpreted with isotope fractionation models to constrain the relative rates of nitrification, denitrification, and assimilation.

Units & currency

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

Sample size

Typical samples are 30–60 mL in volume.

Limitations

Isotopic signatures reflect the net effect of multiple simultaneous processes and cannot be uniquely attributed to individual transformations without additional constraints (rate measurements or models). Multiple nitrate sources with overlapping isotopic compositions can complicate interpretation. Isotope exchange reactions (between δ¹⁸O-NO₃^- and water) can confound oxygen isotope signatures. Models are required to interpret the data quantitatively.

Example Applications & Protocols

Classic examples

• Deb et al. (2024) Microbial nitrogen transformations tracked by natural abundance isotope studies and microbiological methods

Recent applications

• Wankel et al. (2007) Nitrification in the euphotic zone as evidenced by nitrate dual isotopic composition

Common calculations/conversions

• Isotope enrichment factor ε = (δ¹⁵N_substrate − δ¹⁵N_product); typical ε for denitrification: −15 to −30‰; for nitrification: −14 to −38‰.