McPaul Katelyn, Wankel Scott D, Seltzer Alan M
MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering, Cambridge and Woods Hole, Massachusetts, USA.
Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA.
Rapid Commun Mass Spectrom. 2025 Oct 15;39(19):e10094. doi: 10.1002/rcm.10094.
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.
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.
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.
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.
水中溶解二氮气体（δN-N）的同位素组成能够有力地限制水生系统中氮循环的来源和路径。然而，由于这些系统中大量存在源自大气的溶解氮，因此需要高精度（约0.001‰量级）的氮同位素测量并结合惰性气体测量，以区分大气信号和生物地球化学信号。此外，在这种精度水平下，氮的溶解度平衡同位素分馏及其对温度和盐度的依赖性尚未得到充分约束。
我们引入了一种用于样品采集、处理和动态双进样质谱分析的新技术，能够高精度测量水中的δN-N和δ(N/Ar)，同时测量δ(Ar/Ar)和δ(Kr/N)。我们评估了该技术的重现性，并利用它重新确定了一系列温度和盐度条件下溶解氮的溶解度平衡同位素效应。
我们的技术实现了δN-N（0.006‰）和δ(N/Ar)（0.41‰）的测量重现性（1σ），适用于追踪水生环境中的生物地球化学氮循环。通过一系列气-水平衡实验，我们发现水中氮的溶解度平衡同位素效应（ε = α/1000 - 1，其中α = (N/N)/(N/N)）为ε(‰) = 0.753 - 0.004•T，其中T为温度（°C），在约2°C - 23°C的温度范围和约0 - 30 psu的盐度范围内，不确定性约为0.001‰。我们发现ε对盐度没有明显依赖性。
我们的新方法能够在同一样品中高精度测量溶解氮和氩的同位素组成，以及溶解的N/Ar和Kr/N比值。将氮与惰性气体的测量相结合，有助于量化生物地球化学来源的过量氮及其同位素组成。该方法在海洋、沿海和淡水环境中有广泛应用，可用于表征和定量约束潜在的氮循环来源和路径，并区分这些系统中的物理和生物同位素信号。
