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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 atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biolog ...
using the heavier, stable
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at sevent ...
isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers ...
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 scientific discipline that involves the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment (including the biosphere, the cryosphere, t ...
,
soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to t ...
,
environmental science Environmental science is an interdisciplinary academic field that integrates physics, biology, and geography (including ecology, chemistry, plant science, zoology, mineralogy, oceanography, limnology, soil science, geology and physical geograp ...
,
environmental microbiology A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scal ...
and small molecule activation research.


Applications

15N tracing allows researchers to distinguish specific nitrogen conversions from a network of simultaneous reactions; e.g.
ammonium The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary ...
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 or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...
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 nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...
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 atomi ...
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 ''Ntrace'' model based on a
Markov chain Monte Carlo In statistics, Markov chain Monte Carlo (MCMC) methods comprise a class of algorithms for sampling from a probability distribution. By constructing a Markov chain that has the desired distribution as its equilibrium distribution, one can obtain ...
simulation.


References

{{reflist * * * * Nitrogen cycle