Nitrogen stable isotopes are indicators used to identify the source types of nitrates in water from mineral fertiliser or organic fertiliser (Xue et al., 2009). Nitrogen has three isotopes (13N, 14N, and 15N). 13N is a radioisotope of which half-life is around 10 minutes. 14N is the most abundant stable isotope (99.6%). The N stable isotopes can be used as an indicator because the ratios of 15N to 14N slightly differ depending on the sources of N. Stable isotope ratios are expressed as the deviation (δ15N; unit ‰) from the international standard ratio.
Xue et al. (2009) compiled previously reported δ15N-NO3 values and evaluated the potential of N stable isotopes as a N source indicator. They reported that different N sources show different δ15N-NO3 values. For example, δ15N-NO3 of mineral fertilisers such as ammonium fertiliser, nitrate fertiliser, and urea varied between -6 ‰ to +6‰. Manure and sewage showed high δ15N-NO3 values: typical δ15N-NO3 values of manure were between +5‰ to +25 ‰ and those of sewage were between +4 ‰ to +19 ‰.
When a single N source dominates, δ15N-NO3 can be a useful tool (Xue et al., 2009). However, when multiple sources are present, the δ15N-NO3 interpretation can be highly uncertain because isotope fractionation occurs during the N cycles through various soil and microbial reactions. To overcome the uncertainty, combining with other isotopes such as δ18N-NO3 or boron isotopes (δ11B) are recommended.