The main source of nitrogen is the air; the 15N/14N isotope ratio of the air (free of anthropogenic pollutants) has been chosen as the standard (see Table 3.6 earlier in this chapter). In addition, the biosphere also contains a significant amount of nitro­gen. Nitrogen is not frequently observed in the rocks because the nitrates usually dissolve in water. The nitrate in water, however, is toxic. The S15N value can give information on the origin of the polluting sources of nitrate, assuming that the nitrogen isotope ratios are different and that neither isotope exchanges nor chemical reactions take place between the different sources of nitrate.

The sources of nitrate can include the following:

• The nitrogen content of soils, including all nitrogen compounds. The characteristic value of S15N soils is in the range of 15m to 19m.

• The nitrate content of the soil, S15N is 12m to 19m. This value shows that the abundance of 15N of the nitrate in soil can be lower than the mean value of S15N.

• The fresh excrement of animals typically has S15N in the range of 11 and 16m; however, for example, penguin excrement shows S15N^18m. When aging, ammonia, with the lighter isotope (14N), evaporates because the partial pressure of ammonia containing the lighter isotope is higher. Therefore, 615N increases up to 110m to 123m. In the soil of the rookeries, 515N is even higher, and in the soil of a penguin rookery, it can reach more than 130m.

• Synthetic fertilizers have 615N = 12m to 17m. This value can be explained by the fact that the fertilizers are synthesized from air (615N = 0) and mineral nitrogen sources with much higher 615N. In addition, the chemical isotope effects during the production (i. e., the contact catalytic synthesis of ammonia) can also influence the isotope ratio.

• When the nitrate content of the fertilizers (including organic and synthetic) by the evapora­tion of ammonia decreases by 20%, the 615N increases by 5m. Since the nitrogen isotope ratios are different in the original organic and inorganic fertilizers, a given value of 515N can relate to different polluting sources. For example, in sandy soil, 515N = 14m to 15m may show that the polluting source is synthetic fertilizer, while in clayey soil, the same value can mean that the pollution originates from organic fertilizer. Therefore, the nitrogen isotope ratio alone gives no definite information on the polluting sources.

The ratio of 15N/14N presents a characteristic distinction between herbivores and carnivores, as the 15N isotope tends to be concentrated by 3—4m with each step of the food chain (terrestrial plants, with the exception of legumes, has the isotopic ratio 2—6m of N). Measuring the nitrogen isotope ratio in hair, for example, can give archeological information on alimentary habits.