RATIONALE Natural abundance 15N values in soil samples analysed by isotope ratio mass spectrometry (IRMS) are often used to confirm a perturbation after it has been indicated by other parameters. We propose a concept of how 15N values may a priori indicate a perturbation. METHODS We analysed the 15N values and C:N ratios of 102 soil samples from five regions in northern Eurasia by using an elemental analyser coupled to an isotope ratio mass spectrometer. RESULTS Unperturbed samples ranged in 15N values from 4.8 to 9.7 parts per thousand and in C:N ratio from 6.4 to 48.1. The 15N values were linearly proportional to the inverse of the square root of the C:N ratio (R2=0.79). At any particular C:N ratio, 94 % of the 15N values of the unperturbed samples were within +/- 2.4 parts per thousand, but 72 % of perturbed samples had values outside this range. CONCLUSIONS The 15N natural abundance values, when related to the C:N ratios, may readily indicate perturbation of soil N cycling prior to other, more demanding investigations into related processes and extend the current use of IRMS in ecosystem research.