Isotopologue enrichment factors of N 2 O reduction in soils

Isotopic signatures can be used to study sink and source processes of N 2 O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N 2 O‐to‐N 2 step of denitrification ( ε ) in two arable soils, a silt‐loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the ε of 18 O ( ε 18O ) and of average 15 N ( ε bulk ), the ε of the 15 N site preference within the linear N 2 O molecule ( ε SP ) was also determined. Soils were anaerobically incubated in gas‐tight bottles with N 2 O added to the headspace to induce N 2 O reduction. Pre‐treatment included the removal of NO   3 −to prevent N 2 O production. Gas samples were collected regularly to determine the dynamics of N 2 O reduction, the time course of the isotopic signatures of residual N 2 O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N 2 O concentration and anaerobic pre‐incubation with or without addition of N 2 O. N 2 O reduction rates were affected by the soil type and initial N 2 O concentration. The ε 18O and ε bulk ranged between −13 and −20‰, and between −5 and −9‰, respectively. Both quantities were more negative in the Gleyic Podzol. The ε of the central N position ( ε α ) was always larger than that of the peripheral N‐position ( ε β ), giving ε SP of −4 to −8‰. The ranges and variation patterns of ε were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between ε 18O and ε bulk which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable ε under certain conditions suggests that these values could serve to identify N 2 O reduction from the isotopic fingerprints of N 2 O emitted from any soil. Copyright © 2009 John Wiley & Sons, Ltd.