1. The mechanism of oxygen isotope fractionation during N2O production by denitrification.
- Author
-
Lewicka-Szczebak, D., Dyckmans, J., Kaiser, J., Marca, A., Augustin, J., and Well, R.
- Subjects
OXYGEN isotopes ,DENITRIFICATION ,NITRATE reductase ,NITRIC oxide reduction ,SOIL moisture - Abstract
The isotopic composition of soil-derived N
2 O can help differentiate between N2 O production pathways and estimate the fraction of N2 O reduced to N2. Until now, δ18 O of N2 O has been rarely used in the interpretation of N2 O isotopic signatures because of the rather complex oxygen isotope fractionations during N2 O 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 (H2 O), 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 N2 O production by denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the δ18 O values of the produced N2 O. We performed several soil incubation experiments. For the first time, Δ17O isotope tracing was applied to simultaneously determine the extent of oxygen isotope exchange and any associated oxygen isotope effect. We found bacterial denitrification to be typically associated with almost complete oxygen isotope exchange and a stable difference in δ18 O between soil water and the produced N2 O of δ18 O(N2 O/H2 O) = (17.5 ± 1.2) ‰. However, some experimental setups yielded oxygen isotope exchange as low as 56% and a higher δ18 O(N2 O/H2 O) of up to 37 ‰. The extent of isotope exchange and δ18 O(N2 O / H2 O) showed a very significant correlation (R2 = 0.70, p < 0.00001). We hypothesise that this observation was due to the contribution of N2 O 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 the isotope exchange occurs prior to the isotope branching and that the mechanism of 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 δ18 O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects. [ABSTRACT FROM AUTHOR]- Published
- 2015
- Full Text
- View/download PDF