Influence of 15 N enrichment on the net isotopic fractionation factor during the reduction of nitrate to nitrous oxide in soil

Nitrous oxide, a greenhouse gas, is mainly emitted from soils during the denitrification process. Nitrogen stable‐isotope investigations can help to characterise the N 2 O source and N 2 O production mechanisms. The stable‐isotope approach is increasingly used with 15 N natural abundance or relatively low 15 N enrichment levels and requires a good knowledge of the isotopic fractionation effect inherent to this biological mechanism. This paper reports the measurement of the net and instantaneous isotopic fractionation factor ( α   s/p i ) during the denitrification of NO   3 −to N 2 O over a range of 15 N substrate enrichments (0.37 to 1.00 atom% 15 N). At natural abundance level, the isotopic fractionation effect reported falls well within the range of data previously observed. For 15 N‐enriched substrate, the value of α   s/p iwas not constant and decreased from 1.024 to 1.013, as a direct function of the isotopic enrichment of the labelled nitrate added. However, for enrichment greater than 0.6 atom% 15 N, the value of α   s/p iseems to be independent of substrate isotopic enrichment. These results suggest that for isotopic experiments applied to N 2 O emissions, the use of low 15 N‐enriched tracers around 1.00 atom% 15 N is valid. At this enrichment level, the isotopic effect appears negligible in comparison with the enrichment of the substrate. Copyright © 2007 John Wiley & Sons, Ltd.