From these principal N-transforming reactions arises the distribution of isotopes that we observe in nitrate and ammonia in groundwaters. For simplicity, let’s start with the atmospheric N₂ reservoir, which is defined as the ¹⁵N standard (d¹⁵N_N2 = 0‰ AIR). Organic nitrogen fixation has only a minor fractionation effect on ¹⁵N, which is depleted in the fresh organic matter by 1 to 5‰ (Létolle, 1980). The manufacture of urea fertilizers from atmospheric N₂ is also accompanied by only minor fractionation (Fig. 6-12).
 

The fractionation of ¹⁵N through the food web is proportional to the trophic level of the organisms (Minagawa and Wada, 1984). Initially low ¹⁵N contents in algae and other primary producers are magnified by over 10‰ by subsequent consumers through the food chain. The catabolic reaction (destructive metabolism) of amino acids (organic N) in soils as well as in animals produces NH₄⁺, which is depleted in ¹⁵N by several permil. In the case of animals, this imparts a reciprocal enrichment on the solid waste (manure). Nitrification of NH₄⁺ adds a further fractionation, producing NO₃^– that is up to 10‰ more depleted. Complicating the story are the fractionation effects of volatilization, which favour ¹⁴NH₃ and enrich the residual ammonium solution.
 

Denitrification proceeds with a series of intermediary steps involving various nitrogen oxides. Net fractionations up to e¹⁵N_N2-NO3 = –20‰ were observed by Wada et al. (1975) and Létolle (1980). In groundwaters of the "Fuhrberger Feld" catchment (case study below), Böttcher et al. (1990) determined a value of –15.9‰.

The d¹⁸O composition of nitrate adds another perspective on the origin of NO₃^–. Experimental work has shown that in biologically formed nitrate, only one oxygen atom comes from atmospheric O₂. The other two come from the water (e.g. Hollocher, 1984), which is considerably more depleted in ¹⁸O. This contrasts with nitrate in synthetic fertilizers, which receives its oxygen primarily from atmospheric O₂. No significant ¹⁸O-fractionation effects appear to accompany nitrification. Since the two oxygen sources differ significantly, "natural" and "synthetic" nitrate should show differing isotopic compositions (Fig. 6-12). This is not always the case (Amberger and Schmidt, 1987), although d¹⁸O_NO3 has proven to be an excellent tracer of denitrification. Böttcher et al. (1990) found a remarkably consistent Rayleigh enrichment, from which they calculated e¹⁸O_product-NO3 = –8.0‰. Together with d¹⁵N_NO3, these are excellent tools to demonstrate denitrification in groundwaters (Fig. 6-12). Where denitrification proceeds via the oxidation of organic carbon, a reciprocal d¹³C depletion in the DIC should be observed.
 

Fig. 6-12 The isotopic composition of various sources of NO₃^–. The enrichment trend for NO₃^– during denitrification is shown with % of original concentration remaining. d¹⁸O values for nitrate vary according to the d¹⁸O of local groundwaters. Ranges are for groundwaters with d¹⁸O_H2O ~ –10‰ VSMOW.