Isotopic measurements---15N-labeled ammonium (15NH4+) accumulation rate in 15NO3- added incubation

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

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DNRA rate (15NH4+ accumulation)
Approach: 15NO3- addition; 15NH4+ accumulation measured by sequential conversion to N2O
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours
Units: µmol N m-2 d-1
Community captured: bulk
Co-measurements: nitrite, nitrate

Method Overview

Dissimilatory nitrate reduction to ammonium (DNRA) is quantified by tracking the appearance of ¹⁵N in the ammonium pool after addition of ¹⁵NO₃^- to anoxic incubations. DNRA bacteria reduce NO₃^- directly to NH₄⁺, bypassing N₂ production. After incubation, the ¹⁵NH₄⁺ fraction in the dissolved ammonium pool is measured using the sequential conversion method of Kuroiwa et al. (2020): ammonium is oxidized to NO₂^- using hypobromite, then converted to N₂O by sodium azide in acetic acid, and the N₂O isotopologue ratios are measured by GC-IRMS.

Scale of measurement

Point samples using anoxic incubations of hours. Most applicable to suboxic or anoxic water layers and sediments where DNRA organisms are active.

Data generated

DNRA rates in µmol N m^-2 d^-1 or nmol N L^-1 d^-1. Combined with denitrification measurements (e.g., N₂ production), the relative competition between DNRA and denitrification as nitrate sinks can be quantified.

Units & currency

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

Sample size

Typical samples are 250 mL to 1 L in volume.

Limitations

Nitrification occurring during the incubation can produce unlabelled NH₄⁺ → ¹⁴NO₂^- → ¹⁴NO₃^-, which dilutes the ¹⁵N pool. Uptake of the produced ¹⁵NH₄⁺ by nitrifying organisms or phytoplankton during the incubation reduces the measurable signal. These isotope dilution effects require correction for accurate rate estimates.

Example Applications & Protocols

Classic examples

• Kuroiwa et al. (2020) Measurement of the potential rates of dissimilatory nitrate reduction to ammonium based on ¹⁴NH₄⁺/¹⁵NH₄⁺ analyses via sequential conversion to N₂O

Recent applications

• Bonaglia et al. (2016) Denitrification and DNRA at the Baltic Sea oxic-anoxic interface

Common calculations/conversions

• DNRA rate (nmol N L^-1 d^-1) = [¹⁵NH₄⁺]_accumulated / atom% ¹⁵N enrichment of NO₃^- pool / incubation time.