15N tracers

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

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Nitrification rate (15N tracer)
Approach: 15N tracer incubation; tracking 15NO2- and 15NO3- production
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: nM d-1
Community captured: bulk
Co-measurements: ambient [NH4+], [NO2-], [NO3-]

Method Overview

¹⁵N-labelled ammonium (¹⁵NH₄⁺) and/or nitrite (¹⁵NO₂^-) are added to dark seawater incubations. Ammonia-oxidizing organisms convert ¹⁵NH₄⁺ to ¹⁵NO₂^-, and nitrite-oxidizing organisms convert ¹⁵NO₂^- to ¹⁵NO₃^-. By tracking the appearance of ¹⁵N in the oxidized DIN pools over time (measured by IRMS after chemical conversion to N₂O), the rates of ammonium oxidation and nitrite oxidation can be determined independently. Combined with dark ¹³C-fixation measurements in the same incubation, the chemoautotrophic production associated with nitrification can also be estimated.

Scale of measurement

Point sample; dark incubations of hours to days. Must be performed in strict darkness to prevent phytoplankton uptake of ¹⁵N.

Data generated

Ammonium oxidation rates and nitrite oxidation rates separately (nM d^-1). The ratio of ammonium to nitrite oxidation rates reveals whether the two processes are tightly coupled or spatially decoupled in the water column.

Units & currency

Units are nM d^-1 (nmol N L^-1 d^-1). The currency is nitrogen.

Sample size

Typical samples are 250 mL to 1 L in volume.

Limitations

The tracer additions must be at sufficiently low concentrations to avoid perturbing the ambient substrate pool; this is especially challenging in oligotrophic waters where ambient [NH₄⁺] is in the low-nM range. Isotope dilution from ongoing regeneration of unlabelled NH₄⁺ can cause underestimation. Dark incubation requirements can alter the microbial community and substrate dynamics.

Example Applications & Protocols

Classic examples

• Ward (2011) Measurement and distribution of nitrification rates in the oceans

Common calculations/conversions

• Ammonium oxidation rate = d(¹⁵NO₂^-) / dt / atom% ¹⁵N-enrichment of NH₄⁺ pool.
• Nitrite oxidation rate = d(¹⁵NO₃^-) / dt / atom% ¹⁵N-enrichment of NO₂^- pool.