Isotopologue effects during N₂O reduction in soils and in pure cultures of denitrifiers

Abstract

Site preference (SP), the difference in δ 15 N between the central and outer nitrogen atoms in N 2 O, is a powerful approach for apportioning fluxes of N 2 O from soils to nitrification and denitrification (Sutka et al., 2006). A critical aspect of the use of SP data to apportion sources of N 2 O to nitrification and denitrification is the need to evaluate data for isotope shifts that may have occurred during N 2 O reduction in soils prior to its escape to the atmosphere. We present data on the isotopologue effects during reduction of N 2 O during anaerobic incubation of soils and pure cultures of denitrifying bacteria. Isotopic enrichment factors for N 2 O reduction in soil mesocosms experiments varied between −9.2 and −1.8‰ for nitrogen and between −25.1 and −5.1‰ for oxygen. In pure cultures of Psuedomonas stutzeri and Psuedomonas denitrificans we observed isotopic enrichment factors for SP of −5.0 and −6.8‰, respectively. We further find that N 2 O consumption produces consistent relationships between δ 18 O and δ 15 N and δ 18 O and the δ 15 N of the central N atom in N 2 O of 2.5 and 1.6, respectively, which are clearly diagnostic of this process. Our results indicate that SP may be altered during reduction of N 2 O and thus bias evaluations of its origins. To understand the impacts of N 2 O reduction in soil flux studies on source isotope signals we modeled the isotope effects of N 2 O production occurring simultaneous with reduction and find increasingly curvilinear relationships between δ 18 O and δ 15 N and δ 18 O and δ 15 N α with increased reduction. Consequently, a deviation from the linear mixing relationship between soil‐derived and atmospheric N 2 O is an indication of extensive reduction. On the basis of our characterization of isotopic fractionation during N 2 O reduction, we show that the rate of reduction would have to be substantially greater than 10% of that of production to impact SP estimates of N 2 O from denitrification by more than a few percent. Nonetheless, reduction results in a small, but potentially important, increase in SP away from values proposed for bacterial denitrification (0‰) toward those associated with production from nitrification (33‰) (Sutka et al., 2006). On this basis, estimates of the proportion of N 2 O derived from denitrification obtained from SP values are underestimates and therefore conservative.