Nitrogen-15 (¹⁵N) tracing is a technique to study the

nitrogen cycle The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmosphere, atmospheric, terrestrial ecosystem, terrestrial, and marine ecosystems. The conversion of nitrogen can ...

using the heavier, stable

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

¹⁵ N. Despite the different weights, ¹⁵N is involved in the same chemical reactions as the more abundant ¹⁴N and is therefore used to trace and quantify conversions of one nitrogen compound to another. ¹⁵N tracing is applied in

biogeochemistry Biogeochemistry is the Branches of science, scientific discipline that involves the study of the chemistry, chemical, physics, physical, geology, geological, and biology, biological processes and reactions that govern the composition of the natu ...

soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, soil classification, classification and Soil survey, mapping; Soil physics, physical, Soil chemistry, chemical, Soil biology, biologica ...

, environmental science,

environmental microbiology ''Environmental Microbiology'' is a monthly peer-reviewed scientific journal focused on microbial interactions and microbial processes in the environment. It is published by Wiley-Blackwell. Until January 2024, it was an official journal of the S ...

and small molecule activation research.

Applications

¹⁵N tracing allows researchers to distinguish specific nitrogen conversions from a network of simultaneous reactions; e.g.

ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...

can at the same time be oxidised by autotrophic microorganisms, produced by mineralisation of organic matter, produced by dissimilatory nitrate reduction and assimilated by microbes and plants. In this case, quantifying the absolute amounts of ammonium does not explain how it is produced or consumed. However, the conversion of one ¹⁵N labelled compound to another can directly be linked through the isotopic signature. ¹⁵N tracing has been applied to quantify rates of nitrogen transformations in

soil Soil, also commonly referred to as earth, is a mixture of organic matter, minerals, gases, water, and organisms that together support the life of plants and soil organisms. Some scientific definitions distinguish dirt from ''soil'' by re ...

and to distinguish the sources of the greenhouse gas

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or factitious air, among others, is a chemical compound, an Nitrogen oxide, oxide of nitrogen with the Chemical formula, formula . At room te ...

under different environmental conditions.

Methodical approaches

The two main approaches are

natural abundance In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the ato ...

and enrichment techniques.

Natural abundance techniques

Natural abundance techniques can be applied without artificial disturbance. The natural ¹⁵N abundances are expressed in delta (δ) notation relative to the ¹⁵N concentration in the air. Due to enzymatic discrimination, natural ¹⁵N abundances change slightly in microbially mediated reactions in soil. Apart from δ values, the site preference of ¹⁵N and ¹⁴N (isotopomers) for the inner or outer position within the nitrous oxide molecule has been used to determine its sources (nitrification or denitrification).

Enrichment techniques

When nitrogen substrates are artificially enriched (labeled) with ¹⁵N, the product of a reaction can directly be linked to its substrate. In contrast to natural abundance techniques, ¹⁵N labeling allows to precisely calculate reaction rates. The amendment of additional nitrogen can also be a bias by changing natural nitrogen transformations. In agricultural soil, however, application of ¹⁵N enriched tracers, such as ammonium and nitrate, resembles conventional fertilisation practise. A way to calculate nitrogen transformation rates in soil can be achieved by numerical approximation that takes different, simultaneous nitrogen transformations into account. A numerical tool to study the nitrogen cycle is the ''N_trace'' model based on a

Markov chain Monte Carlo In statistics, Markov chain Monte Carlo (MCMC) is a class of algorithms used to draw samples from a probability distribution. Given a probability distribution, one can construct a Markov chain whose elements' distribution approximates it – that ...

simulation.

References

{{reflist * * * * Nitrogen cycle