instrumental techniques applied to mineralogy and geochemistry

106 Clemente Recio
although 15 N depends directly on the N source, exchange with soil-N can affect its
isotopic signature (Komor and Anderson, 1993). An even larger effect is associated to
ammonium volatilization (Mariotti et al., 1988; Bötcher et al., 1990). It is therefore
necessary to establish the isotopic relationships within the context of the local conditions
of the area under investigation. A multielement / multisotopic study will help in this
regard. For example, the additional determination of 18 O allows characterisation of
possible processes that might potentially modify the concentration and the isotopic ratios
of N in nitrates, such as denitrification (Amberger and Schmidt, 1987; Böttcher et al.,
1990).
The few remaining uncertainties after N and O isotopic analysis can be clarified by
the use of additional isotopic systems (S and O in sulphates; O in phosphates, … etc) and
chemical data (for example, Böhlke and Denver, 1995). Typical 15 N values are between
–5 to +3,5‰ in soils where chemical fertilizers have been applied; natural soil organic
matter has 15 N +3,5 to +7,5‰; animal manure is between +10 to +20‰, and
downstream of septic tanks values around 15 N+10‰ (Aravena et al., 1993) to
15 N+8‰ (Kreitler et al., 1978) have been measured.
FIGURE 9. O and N isotope characterisation of various sources and processes that can contribute
nitrate pollution to soil and water. From Bleifuss et al. (2001).