Abstract
Abstract. The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2. Until now, δ18O of N2O has been rarely used in the interpretation of N2O isotopic signatures because of the rather complex oxygen isotope fractionations during N2O production by denitrification. The latter process involves nitrate reduction mediated through the following three enzymes: nitrate reductase (NAR), nitrite reductase (NIR) and nitric oxide reductase (NOR). Each step removes one oxygen atom as water (H2O), which gives rise to a branching isotope effect. Moreover, denitrification intermediates may partially or fully exchange oxygen isotopes with ambient water, which is associated with an exchange isotope effect. The main objective of this study was to decipher the mechanism of oxygen isotope fractionation during N2O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18O values of the produced N2O. In our soil incubation experiments Δ17O isotope tracing was applied for the first time to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found that N2O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100 % and a stable difference between the 18O ∕ 16O ratio of soil water and the N2O product of δ18O(N2O ∕ H2O) = (17.5 ± 1.2) ‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56 % and a higher δ18O(N2O ∕ H2O) of up to 37 ‰. The extent of isotope exchange and δ18O(N2O ∕ H2O) showed a significant correlation (R2 = 0.70, p < 0.00001). We hypothesize that this observation was due to the contribution of N2O from another production process, most probably fungal denitrification. An oxygen isotope fractionation model was used to test various scenarios with different magnitudes of branching isotope effects at different steps in the reduction process. The results suggest that during denitrification, isotope exchange occurs prior to isotope branching and that this exchange is mostly associated with the enzymatic nitrite reduction mediated by NIR. For bacterial denitrification, the branching isotope effect can be surprisingly low, about (0.0 ± 0.9) ‰, in contrast to fungal denitrification where higher values of up to 30 ‰ have been reported previously. This suggests that δ18O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects.
Highlights
Our ability to mitigate soil N2O emissions is limited due to poor understanding of the complex interplay between N2O production pathways in soil environments
The isotopic composition of soil-derived N2O can help differentiate between N2O production pathways and estimate the fraction of N2O reduced to N2
Isotopocule analyses of N2O, including δ18O, average δ15N (δ15Nav) and 15N site preference within the linear N2O molecule (δ15Nsp) have been used for several years to help differentiate between N2O production pathways (Opdyke et al, 2009; Perez et al, 2006; Sutka et al, 2006; Toyoda et al, 2005; Well et al, 2008), the various microbes involved (Rohe et al, 2014a; Sutka et al, 2003, 2008) and to estimate the fraction of N2O reduced to N2 (Ostrom et al, 2007; Park et al, 2011; Toyoda et al, 2011; Well and Flessa, 2009)
Summary
Our ability to mitigate soil N2O emissions is limited due to poor understanding of the complex interplay between N2O production pathways in soil environments. We need to recognize the isotope effects associated with nitrate and N2O reduction to quantify the entire gaseous nitrogen losses as N2O and N2 based on the N2O isotopic signatures (Lewicka-Szczebak et al, 2014, 2015). This would be most effective if either of the isotopic signatures (δ18O, δ15Nav or δ15Nsp) were stable or predictable for N2O produced by each of the relevant N2O forming processes (e.g. heterotrophic bacterial denitrification, fungal denitrification, nitrifier denitrification and nitrification). We hypothesize that this could be the case for δ18O, and this study aims to increase the understanding of the factors controlling δ18O during N2O production in soils
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