Quantifying NO reduction to N during denitrification in soils via isotopic mapping approach: Model evaluation and uncertainty analysis.

The last step of denitrification, i.e. the reduction of NO to N, has been intensively studied in the laboratory to understand the denitrification process, predict nitrogen fertiliser losses, and to establish mitigation strategies for NO. However, assessing N 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 δN/δO slope that has been proposed to potentially be used to indirectly quantify NO reduction to N at field or larger spatial scales. We evaluate the model using data obtained from seven independent soil incubation studies conducted under a He-O 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 NO 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 NO 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 NO isotope signatures.