15N tracer-based method

Template:BreadcrumbsNutrients

Denitrification rate (¹⁵N isotope pairing)
Approach: ¹⁵N-nitrate addition; ¹⁵N₂ and ¹⁵N₂O production measured by MIMS/GC-IRMS
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours
Units: µmol N m^-2 d^-1; nmol N L^-1 d^-1
Community captured: bulk
Co-measurements: nitrate and nitrite concentrations, N₂ and N₂O concentrations

Method Overview

The ¹⁵N isotope pairing technique (IPT) measures denitrification directly by tracking the production of ¹⁵N-labelled dinitrogen gas. ¹⁵N-nitrate (K¹⁵NO₃) is added to anoxic or suboxic incubations, and denitrifiers reduce the labelled nitrate to N₂ and N₂O. Because both ¹⁴NO₃^- and ¹⁵NO₃^- are present, the N₂ produced contains three isotopologue combinations: ¹⁴N¹⁴N (from two unlabelled nitrate), ¹⁴N¹⁵N (from one of each), and ¹⁵N¹⁵N (from two labelled nitrate). The ratio of these isotopologues, measured by MIMS (membrane inlet mass spectrometry) or GC-IRMS, allows total denitrification to be calculated from the labelled fraction alone^[1]. This approach provides direct, unambiguous measurement of N₂ production without relying on proxy gases or inhibitors.

Scale of measurement

Point sample; short incubations (hours) under suboxic or anoxic conditions. The method can be applied to water column samples or sediment slurries.

Data generated

Denitrification rates in nmol N₂-N L^-1 d^-1 or µmol N m^-2 d^-1 (for sediment measurements). The ¹⁵N₂O fraction provides a simultaneous estimate of incomplete denitrification to N₂O.

Units & currency

Units are nmol N₂-N L^-1 d^-1 or µmol N m^-2 d^-1. The currency is nitrogen.

Sample size

Typical samples are 250 mL to 1 L in volume.

Repositories & databases

Limitations

The method assumes that isotope pairing patterns follow a binomial distribution, which requires complete mixing of labelled and unlabelled nitrate and uniform access by denitrifying organisms to both substrate pools. Anammox organisms also produce ¹⁵N-labelled N₂ from the added ¹⁵NO₃^-/¹⁵NO₂^-, which can overestimate denitrification rates if not corrected for. Isotope dilution from unlabelled nitrate regeneration during the incubation reduces sensitivity.

Example Applications & Protocols

Classic examples

Steingruber et al. (2001) Measurement of denitrification in sediments with the ¹⁵N isotope pairing technique ^[1]

Recent applications

Bourbonnais et al. (2024) Denitrification in oxygen minimum zones ^[2]

Common calculations/conversions

Total denitrification rate D_total = D₁₄ + D₁₅; where D₁₅ = (¹⁴N¹⁵N + 2 × ¹⁵N¹⁵N) / 2 × atom% ¹⁵N enrichment.

References

↑ ^1.0 ^1.1 Steingruber, S. M., Friedrich, J., Gächter, R., & Wehrli, B. (2001). Measurement of denitrification in sediments with the ¹⁵N isotope pairing technique. Applied and Environmental Microbiology, 67(9), 3771–3778. https://doi.org/10.1128/AEM.67.9.3771-3778.2001
↑ 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

[Steingruber2001-1] 1.0 ^1.1 Steingruber, S. M., Friedrich, J., Gächter, R., & Wehrli, B. (2001). Measurement of denitrification in sediments with the ¹⁵N isotope pairing technique. Applied and Environmental Microbiology, 67(9), 3771–3778. https://doi.org/10.1128/AEM.67.9.3771-3778.2001

[Bourbonnais2024-2] 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

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