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]

Two very similar expressions for δ¹⁵N are in wide use in hydrology.^[2] Both have the form ${\displaystyle 1000\cdot {\frac {s-a}{a}}}$ ‰ (‰ = 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 ${\displaystyle 1000\cdot {\frac {s-a}{a}}}$ 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 δ¹⁵N.

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 δ¹⁵N.^[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 δ¹⁵N in benthic sediment gives an indication of the contribution of human waste to the total nitrogen in the sediment. Sediment cores analyzed for δ¹⁵N yield an historical record of such waste, with older samples at greater depths.

δ¹⁵N is also used to measure food chain length and the trophic level of a given organism; high δ¹⁵N values are positively correlated with higher trophic levels; likewise, organisms low on the food chain generally exhibit lower δ¹⁵N values. Higher δ¹⁵N values in apex predators generally indicate longer food chains.^[5]

• δ¹³C
• δ¹⁸O
• δ³⁴S
• Isotopic signature
• Isotope analysis
• Isotope geochemistry

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