Oxygen isotope fractionation during N2O production by soil denitrification

Dominika Lewicka-Szczebak, Jens Dyckmans, Jan Kaiser, Alina Marca, Jürgen Augustin, Reinhard Well
2016 Biogeosciences  
<p><strong>Abstract.</strong> The isotopic composition of soil-derived N<sub>2</sub>O can help differentiate between N<sub>2</sub>O production pathways and estimate the fraction of N<sub>2</sub>O reduced to N<sub>2</sub>. Until now, <i>δ</i><sup>18</sup>O of N<sub>2</sub>O has been rarely used in the interpretation of N<sub>2</sub>O isotopic signatures because of the rather complex oxygen isotope fractionations during N<sub>2</sub>O production by denitrification. The latter process involves
more » ... rocess 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 (H<sub>2</sub>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 N<sub>2</sub>O production by soil denitrification and, in particular, to investigate the relationship between the extent of oxygen isotope exchange with soil water and the <i>δ</i><sup>18</sup>O values of the produced N<sub>2</sub>O. <br><br> In our soil incubation experiments Δ<sup>17</sup>O 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 N<sub>2</sub>O formation in static anoxic incubation experiments was typically associated with oxygen isotope exchange close to 100<span class="thinspace"></span>% and a stable difference between the <sup>18</sup>O<span class="thinspace"></span>∕<span class="thinspace"></span><sup>16</sup>O ratio of soil water and the N<sub>2</sub>O product of <i>δ</i><sup>18</sup>O(N<sub>2</sub>O<span class="thinspace"></span>∕<span class="thinspace"></span>H<sub>2</sub>O)<span class="thinspace"></span> = <span class="thinspace"></span>(17.5<span class="thinspace"></span>±<span class="thinspace"></span>1.2)<span class="thinspace"></span>‰. However, flow-through experiments gave lower oxygen isotope exchange down to 56<span class="thinspace"></span>% and a higher <i>δ</i><sup>18</sup>O(N<sub>2</sub>O<span class="thinspace"></span>∕<span class="thinspace"></span>H<sub>2</sub>O) of up to 37<span class="thinspace"></span>‰. The extent of isotope exchange and <i>δ</i><sup>18</sup>O(N<sub>2</sub>O<span class="thinspace"></span>∕<span class="thinspace"></span>H<sub>2</sub>O) showed a significant correlation (<i>R</i><sup>2</sup> = 0.70, <i>p</i> &amp;lt; 0.00001). We hypothesize that this observation was due to the contribution of N<sub>2</sub>O from another production process, most probably fungal denitrification. <br><br> 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<span class="thinspace"></span>±<span class="thinspace"></span>0.9)<span class="thinspace"></span>‰, in contrast to fungal denitrification where higher values of up to 30<span class="thinspace"></span>‰ have been reported previously. This suggests that <i>δ</i><sup>18</sup>O might be used as a tracer for differentiation between bacterial and fungal denitrification, due to their different magnitudes of branching isotope effects.</p>
doi:10.5194/bg-13-1129-2016 fatcat:4kfzxlkggbeszlmw2swfnis554