Quantifying N₂O reduction to N₂ based on N₂O isotopocules – validation with independent methods (helium incubation and ¹⁵N gas flux method)

Abstract. Stable isotopic analyses of soil-emitted N₂O (δ¹⁵N^bulk, δ¹⁸O and δ¹⁵N^sp = ¹⁵N site preference within the linear N₂O molecule) may help to quantify N₂O reduction to N₂, an important but rarely quantified process in the soil nitrogen cycle. The N₂O residual fraction (remaining unreduced N₂O, r_N2O) can be theoretically calculated from the measured isotopic enrichment of the residual N₂O. However, various N₂O-producing pathways may also influence the N₂O isotopic signatures, and hence complicate the application of this isotopic fractionation approach.

Here this approach was tested based on laboratory soil incubations with two different soil types, applying two reference methods for quantification of r_N2O: helium incubation with direct measurement of N₂ flux and the ¹⁵N gas flux method. This allowed a comparison of the measured r_N2O values with the ones calculated based on isotopic enrichment of residual N₂O. The results indicate that the performance of the N₂O isotopic fractionation approach is related to the accompanying N₂O and N₂ source processes and the most critical is the determination of the initial isotopic signature of N₂O before reduction (δ₀). We show that δ₀ can be well determined experimentally if stable in time and then successfully applied for determination of r_N2O based on δ¹⁵N^sp values. Much more problematic to deal with are temporal changes of δ₀ values leading to failure of the approach based on δ¹⁵N^sp values only. For this case, we propose here a dual N₂O isotopocule mapping approach, where calculations are based on the relation between δ¹⁸O and δ¹⁵N^sp values. This allows for the simultaneous estimation of the N₂O-producing pathways' contribution and the r_N2O value.