Quick note on an interesting paper regarding soil N isotopes.
Cohen et al. synthesized about 100 soil samples for C:N and 15N. They found a good relationship between soil C:N and 15N. Beyond this, those soils that were considered "perturbed"were more elevated at a given C:N.
They felt that overgrazing or fire led to enriched 15N when compared at a given C:N.
In conclusion, they state "The d15N natural abundance values, when related to the C:N ratios, may readily indicate perturbation of soil N cycling prior to other..."
Conen, F., M. V. Yakutin, N. Carle, and C. Alewell. 2013. delta(15) N natural abundance may directly disclose perturbed soil when related to C:N ratio. Rapid Communications in Mass Spectrometry 27:1101-1104.