Quantifying N₂O reduction to N₂ during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis

The last step of denitrification, i.e. the reduction of N₂O to N₂, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for N₂O. However, assessing N₂ production via denitrification at large spatial scales is still not possible due to lack of reliable quantitative approaches. Here, we present a novel numerical “mapping approach” model using the δ¹⁵N^sp/δ¹⁸O slope that has been proposed to potentially be used to indirectly quantify N₂O reduction to N₂ at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He–O₂ atmosphere. Furthermore, we analyse the contribution of different parameters to the uncertainty of the model. The model performance strongly differed between studies and incubation conditions. Re-evaluation of the previous data set demonstrated that using soils-specific instead of default endmember values could largely improve model performance. Since the uncertainty of modelled N₂O reduction was relatively high, further improvements to estimate model parameters to obtain more precise estimations remain an on-going matter, e.g. by determination of soil-specific isotope fractionation factors and isotopocule endmember values of N₂O production processes using controlled laboratory incubations. The applicability of the mapping approach model is promising with an increasing availability of real-time and field based analysis of N₂O isotope signatures.