A High-Precision Analytical Technique for Dissolved N Isotopes in Aquatic Systems: Biogeochemical Applications and Determination of Solubility Equilibrium Isotope Effects.

RATIONALE

The isotopic composition of dissolved dinitrogen gas (δN-N) 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 N in these systems, high-precision (on the order of 0.001‰) measurements of N isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N and its temperature and salinity dependence are underconstrained at this level of precision.

METHODS

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

RESULTS

Our technique achieves measurement reproducibility (1σ) for δN-N (0.006‰) and δ(N/Ar) (0.41‰) suitable for tracing biogeochemical nitrogen cycling in aquatic environments. Through a series of air-water equilibration experiments, we find a N solubility equilibrium isotope effect (ε = α/1000 - 1, where α = (N/N)/(N/N)) 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.

CONCLUSIONS

Our new method allows for high-precision measurements of the isotopic composition of dissolved N and Ar, and dissolved N/Ar and Kr/N ratios, within the same sample. Pairing measurements of N with inert gases facilitates the quantification of excess N 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.