A High-Precision Analytical Technique for Dissolved N₂ Isotopes in Aquatic Systems: Biogeochemical Applications and...

The isotopic composition of dissolved dinitrogen gas (δ¹⁵N-N₂) in water can offer a powerful constraint on the sources and pathways of nitrogen cycling in aquatic systems. However, because of the large presence of atmosphere-derived dissolved N₂ in these systems, high-precision (on the order of 0.001‰) measurements of N₂ isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N₂ and its temperature and salinity dependence are underconstrained at this level of precision.

We introduce a new technique for sample collection, processing, and dynamic dual-inlet mass spectrometry allowing for high-precision measurement of δ¹⁵N-N₂ and δ(N₂/Ar) with simultaneous measurement of δ(⁴⁰Ar/³⁶Ar) and δ(Kr/N₂) in water. We evaluate the reproducibility of this technique and employ it to redetermine the solubility equilibrium isotope effects for dissolved N₂ across a range of temperatures and salinities.

Our technique achieves measurement reproducibility (1σ) for δ¹⁵N-N₂ (0.006‰) and δ(N₂/Ar) (0.41‰) suitable for tracing biogeochemical nitrogen cycling in aquatic environments. Through a series of air-water equilibration experiments, we find a N₂ solubility equilibrium isotope effect (ε = α/1000 - 1, where α = (²⁹N₂/²⁸N₂)_dissolved/(²⁹N₂/²⁸N₂)_gas) in water of ε(‰) = 0.753 - 0.004•T where T is the temperature (°C), with uncertainties on the order of 0.001‰ over the temperature range of ~2°C-23°C and salinity range of ~0-30 psu. We find no apparent dependence of ε on salinity.

Our new method allows for high-precision measurements of the isotopic composition of dissolved N₂ and Ar, and dissolved N₂/Ar and Kr/N₂ ratios, within the same sample. Pairing measurements of N₂ with inert gases facilitates the quantification of excess N₂ from biogeochemical sources and its isotopic composition. This method allows for a wide range of applications in marine, coastal, and freshwater environments to characterize and quantitatively constrain potential nitrogen-cycling sources and pathways and to differentiate between physical and biological isotope signals in these systems.