The 2 isotopes of nitrogen, N14 and N15, are both found in various
ecosystems around the planet. N14 is the most common isotope (same chemical
properties, only difference is that N15 has one more neutron in the
nucleus). What you read accounts of in the literature is different ratios of
N15/N14. Various metabolic processes in some organisms "discriminate"
against N15, because each atom weighs slightly more than an atom of N14;
more work to move the N15 around. So, as a result of this continuous
discrimination, some organic residues will be "lighter"; they'll contain
relatively more N14. On the other hand, other metabolic processes in other
organisms don't discriminate against N15, so that they'll contain relatively
more N15 and be considered "heavy" or enriched in N15. The isotopic ratio
can be considered a signature, and gives you important clues to both the
origin of organic material and an idea of how trophic webs work (who eats
who). This is usually analyzed in conjunction with other stable isotope
ratios, usually carbon (C13/C12) and sulfur (S34/S32). It's a very powerful
analytical tool in ecosystem studies, once you have access to the right lab
equipment (atomic absorption spectrophotometers) and set up a meaningful
sampling protocol.
The article you read assumes that _everyone_ knows about stable isotope
ecology, which is kinda hard to believe.
--Bruce Stallsmith
Huntsville, AL
"ask us about our stable isotopes!"
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