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

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From: Biogeosciences(Vol. 18, Issue 3)
Publisher: Copernicus GmbH
Document Type: Article
Length: 441 words

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Abstract :

The prediction of nitrous oxide (N.sub.2 O) and of dinitrogen (N.sub.2) emissions formed by biotic denitrification in soil is notoriously difficult due to challenges in capturing co-occurring processes at microscopic scales. N.sub.2 O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (O.sub.2) supply through diffusion and O.sub.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.sub.2 O and (N.sub.2 O + N.sub.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.sub.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.sub.2 and N.sub.2 O emissions were monitored with gas chromatography. The .sup.15 N gas flux method was used to estimate the N.sub.2 O reduction to N.sub.2. N gas emissions could only be predicted well when explanatory variables for O.sub.2 demand and O.sub.2 supply were considered jointly. Combining CO.sub.2 emission and ansvf as proxies for O.sub.2 demand and supply resulted in 83 % explained variability in (N.sub.2 O + N.sub.2) emissions and together with the denitrification product ratio [N.sub.2 O / (N.sub.2 O + N.sub.2 )] (pr) 81 % in N.sub.2 O emissions. O.sub.2 concentration measured by microsensors was a poor predictor due to the variability in O.sub.2 over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for O.sub.2 demand (SOM) and O.sub.2 supply (diffusivity) reduced the predictive power considerably (60 % and 66 % for N.sub.2 O and (N.sub.2 O+N.sub.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.sub.2 O and (N.sub.2 O + N.sub.2) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving N.sub.2 O flux models and can help to develop mitigation strategies for N.sub.2 O fluxes and improve N use efficiency.

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Gale Document Number: GALE|A651980831