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 are in wide use in hydrology.[2] Both have the form
| 1000 ⋅ | s-a |
| a |
| 1000 ⋅ | s-a |
| a |
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. 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 .
Another application is the assessment of human waste water discharge into bodies of water.[3] The abundance of 15N is greater in human waste water than in natural water sources. Hence in benthic sediment gives an indication of the contribution of human waste to the total nitrogen in the sediment. Sediment cores analyzed for yield an historical record of such waste, with older samples at greater depths.
is also used to measure food chain length and the trophic level of a given organism; high values are positively correlated with higher trophic levels; likewise, organisms low on the food chain generally exhibit lower values. Higher values in apex predators generally indicate longer food chains.[4]