Natural Nitrogen Tracer Methodology
There are four radioactive and two stable isotopes of nitrogen. The
radioactive isotopes have half lives which are too short for most uses, the
half lives ranging from 0.11 seconds to 10.05 minutes. The stable isotopes
14N and l5N are commonly used as tracers. They occur naturally in a
relatively fixed abundance with about 273 atoms of mass 14 for each atom of
mass 15. The chemical properties of 14N and 15N are identical. The
separation of the N isotopes depends on their differences in mass, and
therefore, on physical properties (Hauck, 1973).
The natural variations in the N isotope abundance, designated as    1N
are commonly expressed as parts per thousand differences from the 15N to 14N
ratio in a standard, usually atmospheric N 
2
15  14       15  14
615N -( N/ N)x - (1N!1N)atm x 1000
(15N/14N)atm
15                     15
A positive 6SN indicates a higher    N concentration, and a negative value a
15
lower   N concentration than in atmospheric N . The natural variations in N
2
isotope ratios are small, usually within +10 isotope units, where one isotope
unit is equal to about 0.000037 atom percent 15N (Hauck, 1973).
Previous Studies
Natural variations of nitrogen isotopes have been used to identify
sources of nitrate in surface water (Kohl et at., 1971) but Hauck et al.
(1972), Edwards (1973), Bremmer and Tabatabai (1973) and Hauck (1973)
questioned the method and interpretation of Kohl et al. (1971).    Gormly and
Spalding (1979) considered the above criticism valid as it applies to the
study of Kohl et al. (1971).   However, in Nebraska, Gormly and Spalding (1979)
*       used natural variations of nitrogen isotopes as source indicators of animal
waste versus agricultural fertilizer for areas of coarse-textured soils