Abstract
Denitrification, a microbial process occurring preferentially in Oxygen Deficient Zones (ODZs) of the ocean, transforms bioavailable nitrate (NO₃⁻) into inert nitrogen gas (N₂). Precise measurement of N₂ concentrations in seawater is crucial for understanding this process and quantifying its impact on the marine nitrogen cycle. However, the large pool of N₂ dissolved in seawater of atmospheric origin masks the relatively small additions from denitrification making precise measurements of this biogenic portion difficult. To address this problem, several methods for monitoring N₂ gas production currently exist, including measuring gas pressures and utilizing the ratios of N₂:Ar, O₂:Ar, and PO₄⁻:N₂. In addition, the Isotope Dilution Technique (IDT) is emerging as a promising new approach for validating N₂ measurements. The IDT enables more direct N₂ measurements via mass spectrometry and involves adding a known quantity of ¹⁵N-enriched tracer to seawater samples to determine the concentration of N₂ in the seawater. Excess N₂ beyond the expected background level can then potentially be attributed to denitrification. Laboratory analyses using the IDT with replicates of seawater standards yielded uncertainties of ±1.37 µmol/kg N₂ (0.34%) when using Biological Oxygen Demand (BOD) bottles and ±1.73 µmol/kg N₂ (0.44%) when using Dissolved Gas (DG) bottles. Continued application and improvement of the IDT method can help further decrease the uncertainty below the targeted precision of 0.5%. Additionally, field applications of the IDT were conducted in the Eastern Tropical North Pacific ODZ (January 2022 and March 2023) and Long Island Sound (Summer 2022). At-sea results indicated a slight increase in N₂ concentrations at depths with very low O₂ concentrations, potentially reflecting denitrification. Continued use of the IDT is essential to validate dissolved N₂ concentration measurements and refine our understanding of denitrification processes in marine environments.