δ¹⁵N

In geochemistry, hydrology, paleoclimatology and paleoceanography, δ¹⁵N (pronounced "delta fifteen n") or delta-N-15 is a measure of the ratio of the two stable isotopes of nitrogen, ¹⁵N:¹⁴N.^[1]

Formulas

Two very similar expressions for $δ$ ¹⁵N are in wide use in hydrology.^[2] Both have the form $1000\cdot {\frac {s-a}{a}}$ ‰ (‰ = permil or parts per thousand) where s and a are the relative abundances of ¹⁵N in respectively the sample and the atmosphere. The difference is whether the relative abundance is with respect to all the nitrogen, i.e. ¹⁴N plus ¹⁵N, or just to ¹⁴N. Since the atmosphere is 99.6337% ¹⁴N and 0.3663% ¹⁵N, a is 0.003663 in the former case and 0.003663/0.996337 = 0.003676 in the latter. However s varies similarly; for example if in the sample ¹⁵N is 0.385% and ¹⁴N is 99.615%, s is 0.003850 in the former case and 0.00385/0.99615 = 0.003865 in the latter. The value of $1000\cdot {\frac {s-a}{a}}$ is then 51.05‰ in the former case and 51.38‰ in the latter, an insignificant difference in practice given the typical range of −20 to 80 for $δ$ ¹⁵N.

Applications

The ratio of ¹⁵N to ¹⁴N is of relevance because in most biological contexts, ¹⁴N is preferentially uptaken as the lighter isotope. As a result, samples enriched in ¹⁵N can often be introduced through a non-biological context.

One use of ¹⁵N is as a tracer to determine the path taken by fertilizers applied to anything from pots to landscapes.^[2] Fertilizer enriched in ¹⁵N to an extent significantly different from that prevailing in the soil (which may be different from the atmospheric standard a) is applied at a point and other points are then monitored for variations in $δ$ ¹⁵N^[3].

Another application is the assessment of human waste water discharge into bodies of water.^[4] The abundance of ¹⁵N is greater in human waste water than in natural water sources. Hence $δ$ ¹⁵N in benthic sediment gives an indication of the contribution of human waste to the total nitrogen in the sediment. Sediment cores analyzed for $δ$ ¹⁵N yield an historical record of such waste, with older samples at greater depths.

$δ$ ¹⁵N is also used to measure food chain length and the trophic level of a given organism; high $δ$ ¹⁵N values are positively correlated with higher trophic levels; likewise, organisms low on the food chain generally exhibit lower $δ$ ¹⁵N values. Higher $δ$ ¹⁵N values in apex predators generally indicate longer food chains.^[5]

References

^ Katzenberg, MA (2008). "Chapter 13: Stable Isotope Analysis: A Tool for Studying Past Diet, Demography, and Life History". Biological Anthropology of the Human Skeleton (2nd ed.). Wiley. ISBN 978-0-471-79372-4.
^ ^a ^b Bedard-Haughn, A. (2003). "Tracing $δ$ ¹⁵N through landscapes: potential uses and precautions". Journal of Hydrology. 272 (1–4): 175–190. Bibcode:2003JHyd..272..175B. doi:10.1016/S0022-1694(02)00263-9.
^ Janovský, Martin P.; Ferenczi, Laszlo; Trubač, Jakub; Klír, Tomáš (2024-06-26). "Stable isotope analysis in soil prospection reveals the type of historic land-use under contemporary temperate forests in Europe". Scientific Reports. 14 (1): 14746. doi:10.1038/s41598-024-63563-1. ISSN 2045-2322. PMC 11208554. PMID 38926400.
^ "Sediment Delta 15N". 23 July 2010.
^ Perkins, Matthew J.; McDonald, Robbie A.; Van Veen, F. J. Frank; Kelly, Simon D.; Rees, Gareth; Bearhop, Stuart (27 March 2014). "Application of Nitrogen and Carbon Stable Isotopes (δ15N and δ13C) to Quantify Food Chain Length and Trophic Structure". PLOS One. 9 (3): e93281. Bibcode:2014PLoSO...993281P. doi:10.1371/journal.pone.0093281. PMC 3968125. PMID 24676331.

Formulas

Applications

References

See also