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

The prediction of nitrous oxide (Ninline-formula₂O) and of dinitrogen (Ninline-formula₂) emissions formed by biotic denitrification in soil is notoriously difficult due to challenges in capturing co-occurring processes at microscopic scales. Ninline-formula₂O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (Oinline-formula₂) supply through diffusion and Oinline-formula₂ 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 Ninline-formula₂O and (Ninline-formula₂O inline-formula+ Ninline-formula₂) 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). Oinline-formula₂ 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. COinline-formula₂ and Ninline-formula₂O emissions were monitored with gas chromatography. The inline-formula¹⁵N gas flux method was used to estimate the Ninline-formula₂O reduction to Ninline-formula₂.

N gas emissions could only be predicted well when explanatory variables for Oinline-formula₂ demand and Oinline-formula₂ supply were considered jointly. Combining COinline-formula₂ emission and ansvf as proxies for Oinline-formula₂ demand and supply resulted in 83 % explained variability in (Ninline-formula₂O inline-formula+ Ninline-formula₂) emissions and together with the denitrification product ratio [inline-formulaN₂O inline-formula $M24inlinescrollmathml/$ 8pt14ptsvg-formulamathimgcdb097d754d0791f99b194a2a037445d bg-18-1185-2021-ie00001.svg8pt14ptbg-18-1185-2021-ie00001.png  (inline-formulaN₂O inline-formula+ inline-formulaN₂)] (pr) 81 % in Ninline-formula₂O emissions. Oinline-formula₂ concentration measured by microsensors was a poor predictor due to the variability in Oinline-formula₂ over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for Oinline-formula₂ demand (SOM) and Oinline-formula₂ supply (diffusivity) reduced the predictive power considerably (60 % and 66 % for Ninline-formula₂O and (Ninline-formula₂Oinline-formula+Ninline-formula₂) fluxes, respectively).

The new approach of using X-ray CT imaging analysis to directly quantify soil structure in terms of ansvf in combination with Ninline-formula₂O and (Ninline-formula₂O inline-formula+ Ninline-formula₂) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving Ninline-formula₂O flux models and can help to develop mitigation strategies for Ninline-formula₂O fluxes and improve N use efficiency.