Denitrification in soil as a function of oxygen availability at the microscale

Abstract. The prediction of nitrous oxide (N 2 O) and of dinitrogen (N 2 ) emissions formed by biotic denitrification in soil is notoriously difficult due to challenges in capturing co-occurring processes at microscopic scales. N 2 O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (O 2 ) supply through diffusion and O 2 demand by respiration in the presence of an alternative electron acceptor (e.g. nitrate). This study aimed to explore controlling factors of complete denitrification in terms of N 2 O and (N 2 O  +  N 2 ) fluxes in repacked soils by taking micro-environmental conditions directly into account. This was achieved by measuring microscale oxygen saturation and estimating the anaerobic soil volume fraction (ansvf) based on internal air distribution measured with X-ray computed tomography (X-ray CT). O 2 supply and demand were explored systemically in a full factorial design with soil organic matter (SOM; 1.2 % and 4.5 %), aggregate size (2–4 and 4–8 mm), and water saturation (70 %, 83 %, and 95 % water-holding capacity, WHC) as factors. CO 2 and N 2 O emissions were monitored with gas chromatography. The 15 N gas flux method was used to estimate the N 2 O reduction to N 2 . N gas emissions could only be predicted well when explanatory variables for O 2 demand and O 2 supply were considered jointly. Combining CO 2 emission and ansvf as proxies for O 2 demand and supply resulted in 83 % explained variability in (N 2 O  +  N 2 ) emissions and together with the denitrification product ratio [ N2O /  ( N2O + N2 )] (pr) 81 % in N 2 O emissions. O 2 concentration measured by microsensors was a poor predictor due to the variability in O 2 over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for O 2 demand (SOM) and O 2 supply (diffusivity) reduced the predictive power considerably (60 % and 66 % for N 2 O and (N 2 O + N 2) fluxes, respectively). The new approach of using X-ray CT imaging analysis to directly quantify soil structure in terms of ansvf in combination with N 2 O and (N 2 O  +  N 2 ) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving N 2 O flux models and can help to develop mitigation strategies for N 2 O fluxes and improve N use efficiency.