Denitrification Nitrification abundance KEY WORDSNitrogen isotope fractionation Nitrogen-15 natural SUMMARY A few principles relative to the presentation and use of nitrogen stable isotopic data are briefly reviewed. Some classical relationships between the isotope composition of a substrate undergoing a single-step unidirectional reaction, are introduced.They are illustrated through controlled experiments on denitrification in a soil, and through nitrification by pure cultures of Nitrosomonas europaea. In the latter case, the isotope fractionation is calculated from the isotopic composition of the residual substrate, then of the product and the result is shown to be statistically the same for the two procedures.The isotopic enrichment factor for denitrification is -29.4 + 2.49'0o at 20~ and -24.6 + 0.9%o at 30~ for nitrification this factor is -34.7 • 2.5%0 under the experimental conditions employed.

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