Influence of 15N 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 N2O source and N2O production mechanisms. The stable-isotope approach is increasingly used with 15N natural abundance or relatively low 15N 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) during the denitrification of NO to N2O over a range of 15N substrate enrichments (0.37 to 1.00 atom% 15N). At natural abundance level, the isotopic fractionation effect reported falls well within the range of data previously observed. For 15N-enriched substrate, the value of α was 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% 15N, the value of α seems to be independent of substrate isotopic enrichment. These results suggest that for isotopic experiments applied to N2O emissions, the use of low 15N-enriched tracers around 1.00 atom% 15N 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.