Oxygen isotope fractionation during N<sub>2</sub>O production by soil denitrification

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.

Improved isotopic model based on 15N tracing and Rayleigh‐type isotope fractionation for simulating differential sources of N2O emissions in a clay grassland soil

“…The values of δ 18 O‐N 2 O are determined by NO 3 − , O 2 and soil H 2 O incorporation and reduction effects during the production of N 2 O resulting in 18 O‐depleted or ‐enriched N 2 O, respectively, since the 18 O–N bond is more stable and 16 O is removed more easily from NO 3 − . It is known that oxygen can be incorporated from H 2 O to N 2 O during denitrification to constitute more than 60% of the O in the N 2 O produced‐ . During the first four days of the incubation, the δ 18 O‐N 2 O values increased indicating an independence of the δ 18 O‐N 2 O values from the δ 18 O‐NO 3 values during the production of N 2 O that can be attributed to a lower O‐exchange with water .…”

“…Because the δ 15 N bulk values exhibited a similar upshift until day 4, we assume that this effect is due to an increase in the δ 18 O and δ 15 N values of the NO 3 − precursor resulting from fractionation during intense denitrification in this phase of the experiment (day 4). This would also mean, however, that O‐exchange with water during N 2 O production was incomplete, which has been reported earlier for a dynamic incubation similar to our study …”

“…production was incomplete, which has been reported earlier for a dynamic incubation similar to our study. 45…”

“…− 41,43. It is known that oxygen can be incorporated from H 2 O to N 2 O during denitrification to constitute more than 60% of the O in the N 2 O produced- 44,45. During the first four days of the incubation,the δ 18 O-N 2 O values increased indicating an independence of the δ 18 O-N 2 O values from the δ 18 O-NO 3 values during the production of N 2 O that can be attributed to a lower O-exchange with water.…”

Attribution of N<sub>2</sub>O sources in a grassland soil with laser spectroscopy based isotopocule analysis

“…80 % of produced N 2 O was reduced to N 2 . A correlated increase in the SP, δ 15 N bulk , and δ 18 O values, with SP values poten-tially larger than the endmember value of 32.8 ± 4 ‰, can be explained by N 2 O reduction to N 2 , which is particularly active under wet and anaerobic soil conditions (Wrage et al, 2004;Lewicka-Szczebak et al, 2017). Thus, isotopic fractionation during partial N 2 O reduction must be taken into account in order to apportion isotopic source signatures of soil-emitted N 2 O Verhoeven et al, 2019).…”