High-Precision Measurements of 15N15N, 14N15N, and 14N2 in N2 and Potential Applications to Oceanic Nitrogen Cycle Research

The balance of nitrogen fixation and nitrogen loss in the oceans is uncertain. For example, anaerobic ammonia oxidation could account for 50% or more of marine N₂ production, although its global importance is still poorly known. Isotopic ratios in fixed nitrogen species (e.g., δ¹⁵N and δ¹⁸O values of NO₂^- and NO₃^-) are widely used to trace preservation and removal of N-bearing compounds and/or isotopic variations of their different sources. However, these approaches in general probe only one side of the nitrogen mass balance—the “fixed” nitrogen reservoir—so they offer few constraints on the ultimate loss of nitrogen from that pool as N₂. The rare isotopologue ratio ¹⁵N¹⁵N/¹⁴N₂ in N₂may provide information about those nitrogen-loss processes directly.

We will report the first measurements of Δ₃₀ (the abundance of ¹⁵N¹⁵N relative to that predicted by chance alone), made on a unique high-resolution mass spectrometer (the Nu Instruments Panorama), and we will discuss the potential utility of Δ₃₀ as an independent tracer of the nitrogen cycle. The parameter Δ₃₀ is insensitive to the bulk ¹⁵N/¹⁴N isotopic ratio of the reservoir; instead, it reflects isotopic ordering in N₂, which is altered when N-N bonds are made or broken. Our preliminary measurements of N₂ from denitrifying soils and pure cultures of denitrifiers indicate large kinetic isotopic effects during N-N bond formation that favor ¹⁵N¹⁵N production during denitrification. We also observed a nonstochastic excess of ¹⁵N¹⁵N in tropospheric N₂ [Δ₃₀ = +19.05 ± 0.12‰ (1σ)]. This excess likely comes from fixed-nitrogen loss processes in the biosphere. Variations in Δ₃₀ of N₂ from pure culture experiments (+16.96 to +18.95‰) probably reflect the different isotopic signatures of the enzymes that catalyze denitrification. So, enzyme-specific Δ₃₀ values of dissolved N₂ should provide information about the importance of different biochemical pathways of fixed-nitrogen loss (e.g., denitrification vs. anammox) in the oceans.