text: Nitrogen-15, or N, is often used in agricultural and medical research, for example in the Meselson–Stahl experiment to establish the nature of DNA replication. An extension of this research resulted in development of DNA-based stable-isotope probing, which allows examination of links between metabolic function and taxonomic identity of microorganisms in the environment, without the need for culture isolation. Proteins can be isotopically labelled by cultivating them in a medium containing N as the only source of nitrogen, e.g., in quantitative proteomics such as SILAC.
  Nitrogen-15 is extensively used to trace mineral nitrogen compounds (particularly fertilizers) in the environment. When combined with the use of other isotopic labels, N is also a very important tracer for describing the fate of nitrogenous organic pollutants. Nitrogen-15 tracing is an important method used in biogeochemistry.
  The ratio of stable nitrogen isotopes, N/N or δN, tends to increase with trophic level, such that herbivores have higher nitrogen isotope values than plants, and carnivores have higher nitrogen isotope values than herbivores. Depending on the tissue being examined, there tends to be an increase of 3-4 parts per thousand with each increase in trophic level. The tissues and hair of vegans therefore contain significantly lower δN than the bodies of people who eat mostly meat. Similarly, a terrestrial diet produces a different signature than a marine-based diet. Isotopic analysis of hair is an important source of information for archaeologists, providing clues about the ancient diets and differing cultural attitudes to food sources.
  A number of other environmental and physiological factors can influence the nitrogen isotopic composition at the base of the food web (i.e. in plants) or at the level of individual animals. For example, in arid regions, the nitrogen cycle tends to be more open and prone to the loss of N, increasing δN in soils and plants. This leads to relatively high δN values in plants and animals in hot and arid ecosystems relative to cooler and moister ecosystems. Furthermore, elevated δN have been linked to the preferential excretion of 14N and reutilization of already enriched 15N tissues in the body under prolonged water stress conditions or insufficient protein intake.
  δN also provides a diagnostic tool in planetary science as the ratio exhibited in atmospheres and surface materials "is closely tied to the conditions under which materials form".