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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-221
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-221
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Jul 2020

17 Jul 2020

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This preprint is currently under review for the journal BG.

Denitrification in soil as a function of oxygen supply and demand at the microscale

Lena Rohe1, Bernd Apelt1, Hans-Jörg Vogel1, Reinhard Well2, Gi-Mick Wu3, and Steffen Schlüter1 Lena Rohe et al.
  • 1Helmholtz Centre for Environmental Research – UFZ, Department Soil System Sciences, Theodor-Lieser-5 Str. 4, 06120 Halle, Germany
  • 2Thünen Institute of Climate Smart Agriculture, Bundesallee 65, 38116 Braunschweig, Germany
  • 3Helmholtz Centre for Environmental Research – UFZ, PACE, Permoserstraße 15, 04318 Leipzig, Germany

Abstract. The prediction of nitrous oxide (N2O) and of dinitrogen (N2) emissions formed by biotic denitrification in soil is notoriously difficult, due to challenges in capturing co-occurring processes at microscopic scales. N2O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (O2) supply through diffusion and O2 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 N2O and (N2O+N2) fluxes in repacked soils by taking micro-environmental conditions directly into account. This was achieved by measuring micro-scale oxygen saturation and estimating the anaerobic soil volume fraction (ansvf) based on internal air distribution measured with X-ray computed tomography (X-ray CT). O2 supply and demand was 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 % WHC) as factors. CO2 and N2O emissions were monitored with gas chromatography. The 15N gas flux method was used to estimate the N2O reduction to N2.

N-gas emissions could only be predicted well, when explanatory variables for O2 supply and oxygen demand were considered jointly. Combining ansvf and CO2 emission as proxies of O2 supply and demand resulted in 83 % explained variability in (N2O+N2) emissions and together with the denitrification product ratio [N2O/(N2O+N2)] (pr) 72 % in N2O emissions. O2 concentration measured by microsensors was a poor predictor due to the variability in O2 over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for O2 supply (diffusivity) and O2 demand (SOM) reduced the predictive power considerably (50 % and 58 % for N2O and (N2O+N2) fluxes, respectively).

The new approach of using X-ray CT imaging analysis to directly quantify soil structure in terms of ansvf in combination with N2O and (N2O+N2) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving N2O flux models and can help to develop mitigation strategies for N2O fluxes and improve N use efficiency.

Lena Rohe et al.

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Short summary
This study presents a well-defined and comprehensive experimental setup analyzing total denitrification, i.e. N2O and (N2O+N2) fluxes, in combination with X-ray CT image analysis. (N2O+N2) fluxes were mainly controlled by the interplay of oxygen supply and demand. The former could be estimated by abiotic proxies like the extent of anaerobic soil volumes or diffusivity, whereas the latter was best described by CO2 production or SOM. N2O fluxes additionally depended on the N2O reduction.
This study presents a well-defined and comprehensive experimental setup analyzing total...
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