Natural ¹⁵N abundance in soils

Abstract

According to the classical definition, the isotopic composition or relative abundance of an element is a fixed value, and the behaviour of the light and heavy isotopes in chemical reactions is strictly identical. These are two basic assumptions for conducting tracer experiments with stable isotopes. However, it is now known that the variation in natural abundance of the isotopes of elements is a biogeochemical phenomenon existing universally in nature (Hoering 1955; Hauck 1973). For instance, variations are found in the natural abundance of the stable isotopes of C, H, O, N and S in organisms. Moreover, the behaviour of the light and heavy isotopes involved in various reactions is not strictly identical. The variation in the natural isotopic abundance is caused by isotopic fractionation, also called the mass discriminatory effect, which is brought about by a series of biological, chemical and physical processes in nature and, especially, in living organisms. The so-called mass discriminatory effect refers to the preference of the light isotope of an element ¹⁴N, for example over the heavy one, ¹⁵N, to take part in a reaction. Consequently, in a reaction the newly-formed reaction product is relatively enriched in ¹⁴N, while the remaining reaction substrate becomes enriched in ¹⁵N. The deviation in natural ¹⁵N abundance of a substance from that of the standard (atmospheric N₂) is referred to as δ¹⁵N which is calculated by the equation:

$${\delta ^{15}}N = \frac{{R\left( {sample} \right) - R\left( {standard} \right)}}{{R\left( {standard} \right)}}1000\% $$

where R is the ratio of the ion current intensity of m/e 29 [¹⁵N¹⁴N] to that of m/e 28 [¹⁴N¹⁴N].