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<p>The prediction of nitrous oxide (N<span class="inline-formula">₂</span>O) and of dinitrogen (N<span class="inline-formula">₂</span>) emissions formed by biotic denitrification in soil is notoriously difficult due to challenges in capturing co-occurring processes at microscopic scales. N<span class="inline-formula">₂</span>O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (O<span class="inline-formula">₂</span>) supply through diffusion and O<span class="inline-formula">₂</span> demand by respiration in the presence of an alternative electron acceptor (e.g. nitrate).</p> <p>This study aimed to explore controlling factors of complete denitrification in terms of N<span class="inline-formula">₂</span>O and (N<span class="inline-formula">₂</span>O <span class="inline-formula">+</span> N<span class="inline-formula">₂</span>) 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 (<i>ansvf</i>) based on internal air distribution measured with X-ray computed tomography (X-ray CT). O<span class="inline-formula">₂</span> 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<span class="inline-formula">₂</span> and N<span class="inline-formula">₂</span>O emissions were monitored with gas chromatography. The <span class="inline-formula">¹⁵</span>N gas flux method was used to estimate the N<span class="inline-formula">₂</span>O reduction to N<span class="inline-formula">₂</span>.</p> <p>N gas emissions could only be predicted well when explanatory variables for O<span class="inline-formula">₂</span> demand and O<span class="inline-formula">₂</span> supply were considered jointly. Combining CO<span class="inline-formula">₂</span> emission and ansvf as proxies for O<span class="inline-formula">₂</span> demand and supply resulted in 83 % explained variability in (N<span class="inline-formula">₂</span>O <span class="inline-formula">+</span> N<span class="inline-formula">₂</span>) emissions and together with the denitrification product ratio [<span class="inline-formula">N₂O</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="cdb097d754d0791f99b194a2a037445d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1185-2021-ie00001.svg" width="8pt" height="14pt" src="bg-18-1185-2021-ie00001.png"/></svg:svg></span></span> (<span class="inline-formula">N₂O</span> <span class="inline-formula">+</span> <span class="inline-formula">N₂</span>)] (<i>pr</i>) 81 % in N<span class="inline-formula">₂</span>O emissions. O<span class="inline-formula">₂</span> concentration measured by microsensors was a poor predictor due to the variability in O<span class="inline-formula">₂</span> over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for O<span class="inline-formula">₂</span> demand (SOM) and O<span class="inline-formula">₂</span> supply (diffusivity) reduced the predictive power considerably (60 % and 66 % for N<span class="inline-formula">₂</span>O and (N<span class="inline-formula">₂</span>O<span class="inline-formula">+</span>N<span class="inline-formula">₂)</span> fluxes, respectively).</p> <p>The new approach of using X-ray CT imaging analysis to directly quantify soil structure in terms of ansvf in combination with N<span class="inline-formula">₂</span>O and (N<span class="inline-formula">₂</span>O <span class="inline-formula">+</span> N<span class="inline-formula">₂</span>) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving N<span class="inline-formula">₂</span>O flux models and can help to develop mitigation strategies for N<span class="inline-formula">₂</span>O fluxes and improve N use efficiency.</p>