Scientists have developed a new way to track microbial nitrogen removal using the rare pairing of two heavy nitrogen-15 atoms in N₂ gas molecules.

The article (by Harrison Tasoff of UC Santa Barbara, in collaboration with Holly Ober of UCLA) covers research published in Science on October 8, 2026. Biogeochemists from UC Santa Barbara, UCLA, and other institutions showed that this rare isotopic “clumping” acts as a natural fingerprint for how much nitrogen microbes convert back into gas.

The problem

Nitrogen is essential for life (proteins, DNA), but excess nitrogen compounds in water, from fertilizer runoff, wastewater, etc., can degrade water quality, fuel harmful algal blooms, and create oxygen- depleted “dead zones”.

Microbes help by converting usable nitrogen compounds into N₂ gas that escapes to the atmosphere (via processes like denitrification). Measuring this removal has been difficult because large amounts of atmospheric N₂ dissolve in water and mask the microbial signal.

The clue from two atoms

N₂ consists of two nitrogen atoms. Most are the common isotope nitrogen- 14; a small fraction is the heavier nitrogen-15. Extremely rarely, both atoms in a molecule are nitrogen- 15 (¹⁵N¹⁵N).

• In atmospheric nitrogen, these two heavy atoms pair up more often than random chance would predict (an anomalous “clumped isotope” signature from high-atmosphere photochemistry).
• Microbially produced N₂ has the isotopes paired nearly at random (due to enzyme kinetics).

When microbial N₂ mixes with atmospheric N₂, it dilutes the excess of heavy- heavy pairs. Measuring the relative abundance of this rare molecule therefore reveals how much of the gas came from microbes.

How they measured it

The team used UCLA’s large Panorama high- resolution mass spectrometer, which can separate molecules with tiny mass differences that ordinary instruments cannot resolve.

They analyzed samples from diverse environments: Texas groundwater, lakes in Antarctica and Minnesota, Southern California coastal basins, the Bay of Bengal, and deep- sea sediments off Alaska. Microbial mats at methane seeps, for example, emerged as hotspots for nitrogen conversion.

Lead author Jiarui Liu (who did the work as a postdoc at UCLA and UCSB’s Marine Science Institute) and co- authors including Edward Young (UCLA), David Valentine (UCSB), and Alan Seltzer (University College Dublin) emphasize that while the technique currently requires specialized equipment and is not yet for routine field use, it offers a powerful new geochemical tool for understanding Earth’s nitrogen cycle.

Clumped isotope theory (or clumped- isotope geochemistry) studies the tendency of rare, heavy isotopes to bond preferentially with one another within molecules, rather than being randomly distributed among the more common light isotopes.

Core idea

Ordinary stable- isotope geochemistry measureOrdinary stable-isotope geochemistry measures bulk ratios such as

${}^{15}$N/${}^{14}$N (δ${}^{15}$N)

${}^{13}$C/${}^{12}$C (δ${}^{13}$C)

${}^{18}$O/${}^{16}$O (δ${}^{18}$O)

Clumped- isotope geochemistry goes one step further: it quantifies how often two (or more) rare isotopes occur together in the same molecule, the “clumps.”

Examples of clumped isotopExamples of clumped isotopologues:

In nitrogen gas (N₂): ${}^{15}$N${}^{15}$N (mass- 30)

In CO₂ or carbonate: ${}^{13}$C${}^{18}$O${}^{16}$O (mass- 47)

In methane: ${}^{13}$CH₃D or ${}^{12}$CH₂D₂

Stochastic (random) baseline

If isotopes combined purely by chance, the abundance of a doubly substituted molecule would simply be the product of the individual isotope abundances.

For N₂, the expected stochastic fraction of ${}^{15}$N${}^{15}$N is roughly ${(}^{15}$N abundance${)}^2$.

Why clumping occurs

Bonds between two heavy isotopes have slightly lower zero-point vibrational energy than bonds between light isotopes or mixed pairs. Thermodynamics therefore favor “clumping” of heavy isotopes because it lowers the molecule’s total energy.

This preference is temperature- dependent:

• At low temperatures, the energy difference matters more → more clumping (higher enrichment relative to stochastic).
• At high temperatures, thermal energy overwhelms the small energy difference → the distribution approaches pure randomness.

The deviation from randomness is reported as a Δ value (in per mil, ‰):

$${\mathrm{\Delta }}_i=(\frac{R_{\text{sample}}}{R_{\text{stochastic}}}-1)\times 1000$$Δi​=(Rstochastic​Rsample​​−1)×1000

where R is the ratio of the clumped isotopologue to a reference isotopologue.
Δ = 0 ‰ means a purely stochastic distribution. Positive Δ means excess clumping.

Key applications

1. Paleothermometry (most famous use)
   In carbonate minerals, the amount of ${}^{13}$1C–${}^{18}$O clumping (Δ₄₇) records the temperature at which the mineral formed, independent of the isotopic composition of the water. This has revolutionized paleoclimate reconstruction.
2. Tracing processes (as in the nitrogen paper)
   Atmospheric N₂ has a large excess of ${}^{15}$N${}^{15}$N (Δ₃₀ ≈ +19 ‰) produced by photochemical and ion-molecule reactions high in the atmosphere.
   Microbial processes (denitrification, anammox) produce N₂ with near- stochastic pairing (Δ₃₀ ≈ 0 ‰).
   Mixing of the two sources dilutes the atmospheric excess, allowing scientists to quantify how much N₂ came from microbes.

Other controls on clumping include kinetic isotope effects, mixing of reservoirs with different bulk compositions, diffusion, and certain photochemical or biological fractionations.

Measurement

Because the clumped species are extremely rare (parts per million or less), high -resolution isotope- ratio mass spectrometers (or specialized laser techniques) are required to resolve them from interferences of nearly identical mass.

In short, clumped- isotope theory turns the internal arrangement of isotopes within molecules into a powerful tracer of temperature, reaction pathways, and mixing, information that bulk isotope ratios alone cannot provide.

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Natural ¹⁵N¹⁵N abundances constrain fixed nitrogen loss

Editor’s summary

Nitrogen gas (N₂) is produced naturally by either photochemical reactions in the atmosphere or microbial processes in the biosphere, but the magnitudes of those sources are poorly quantified. Liu et al. showed that the abundance of the rare ${}^{15}$15N${}^{15}$15N isotopologue can be used to determine the amount of biogenic N₂ production in aquatic environments, thereby allowing physical and biological N₂ sources to be distinguished. This approach, applied to a wide spectrum of surface and ground water bodies, will help to better constrain the global nitrogen cycle.
—Jesse Smith

Abstract

Nitrogen regulates primary productivity across much of the biosphere, yet fixed nitrogen loss remains poorly constrained because existing methods rely on indirect proxies or ex situ experiments.

In this study, the authors show that natural abundances of the rare ${}^{15}$N${}^{15}$N isotopologue of dinitrogen (N₂) provide a direct tracer of biological N₂ production across diverse aquatic environments.

• N₂ produced by denitrification and anammox has a near- stochastic ${}^{15}$N${}^{15}$N distribution (≈ 0 ‰).
• Atmospheric N₂ carries a distinct ${}^{15}$N${}^{15}$N excess of ~19 ‰.

This large contrast allows the two sources to be quantitatively distinguished.

Across aquifers, stratified lakes, coastal basins, oxygen minimum zones, and marine sediments, ${}^{15}$N${}^{15}$N measurements reveal widespread nitrogen loss that was previously obscured by physical gas accumulation and nitrogen fixation.

Natural ${}^{15}$N${}^{15}$N abundances therefore provide a general framework for directly constraining fixed nitrogen loss across the aquatic nitrogen cycle.

This is the peer- reviewed paper underlying the popular- science articles.

The full text is behind the Science paywall, but the abstract and editor’s summary capture the core advance:

using the clumped- isotope signature of N₂ (Δ₃₀) as a natural tracer for microbial nitrogen removal.

Journal: Science ,Vol. 394, Issue 6820, pp. 207- 211, 8 October 2026

DOI: 10.1126/science.aei0940

Provided: University of California – Santa Barbara

Authors: Jiarui Liu, David L. Valentine, Annie Bourbonnais, Dale T. Andersen, Daniele Bianchi, Grace Brown, M. Bayani Cardenas, Daniel Fillion, Claudia Frey, Kelsey M. Gosselin, Aoshuang Ji, Franklin S. Kinnaman, Denis Lacelle, Moritz F. Lehman, Katelyn McPa , James Mullahoo, Victoria J. Orph, André Pellerin, Elen Reji, Elizabeth D. Swanner, Tina Treude, Alan M. Seltzer, and Edward D. Young