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

Abstract

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 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). O2 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. 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 demand and O2 supply were considered jointly. Combining CO2 emission and ansvf as proxies for O2 demand and supply resulted in 83 % explained variability in (N2O + N2) emissions and together with the denitrification product ratio [N2O / (N2O + N2)] (pr) 81 % 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 demand (SOM) and O2 supply (diffusivity) reduced the predictive power considerably (60 % and 66 % 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.

KGML-ag: A Modeling Framework of Knowledge-Guided Machine Learning to Simulate Agroecosystems: A Case Study of Estimating N<sub>2</sub>O Emission using Data from Mesocosm Experiments

“…For IMVs, we found oxygen demand indicator (e.g., O2 concentration or flux, CO2 flux, CH4 flux), N mass balance related variables (e.g., N2 flux, soil NO3 -, soil NH4 + , N leaching) and soil water and temperature, can be used to better constrain the processes and therefore improve the KGML performance. Rohe et al (2021) also indicated the importance of O2, CO2 and N2 soil fluxes for N2O predictions. In addition, the layerwise soil observations (e.g., soil NO3 -, soil VWC) at 0-30 cm depth can be used to significantly improve the KGML model quality, according to our feature importance analysis (Fig.…”

Evaluation of denitrification and decomposition from three biogeochemical models using laboratory measurements of N<sub>2</sub>, N<sub>2</sub>O and CO<sub>2</sub>

“…4.2.4). The slow increase in the denitrifier biomass that Coup modeled in the silt-loam soil could be the reason that the modeled ansvf is orders of magnitude smaller than the ansvf measured in another silt-loam soil of similar WFPS (Rohe et al, 2021). This non-realistic, too small and slowly increased denitrifier community led therefore to low N 2 O and N 2 fluxes.…”

Soil aeration and redox potential as function of pore connectivity unravelled by X‐ray microtomography imaging

“…Nevertheless, our study takes place in the air capacity range while thinner pores remained water saturated (Figure S1). Thus, we can neglect O 2 diffusion as it is 10 5 times reduced in the water phase (Rohe et al, 2021). Our technical equipment in conjunction with the large bulk soil volume of 250 cm 3 enabled us to quantify the ε CT within pores up to 15 μm.…”

“…Pot et al (2015) have directly measured pore-scale water configuration at high resolution and demonstrated model predictions of local water-gas phase interfaces using a lattice-Boltzmann approach (Pot et al, 2015). The threedimensional distribution by water and air-filled pores has profound impact on soil reducing conditions, because O 2 diffusion coefficients were drastically curtailed by five orders of magnitude due to water filled-pores (Rohe et al, 2021). Thus, the electrode responds to changes in ε in close vicinity of the Pt surface because it further influences the electron availability due to the presence or absence of O 2 .…”