Oxygen supply and demand as controls of denitrification at the microscale in repacked soil
Authors: Rohe Lena , Schlüter Steffen , Apelt Bernd , Vogel Hans-Jörg , Well Reinhard 
Published: March 2020

&lt;p&gt;The controlling factors of biotic denitrification in soil as a source of the greenhouse gas nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) and of dinitrogen (N&lt;sub&gt;2&lt;/sub&gt;) are still not fully understood due to the challenges in observing processes that co-occur in soil at microscopic scales and the difficulty to measure N&lt;sub&gt;2&lt;/sub&gt; fluxes. N&lt;sub&gt;2&lt;/sub&gt;O production and reduction depend on the extent of anoxic conditions in soil, which in turn are a function of O&lt;sub&gt;2&lt;/sub&gt; supply through diffusion and O&lt;sub&gt;2&lt;/sub&gt; demand by soil respiration in the presence of an alternative electron acceptor (e.g. nitrate).&lt;/p&gt;&lt;p&gt;This study aimed to explore microscopic drivers that control total denitrification, i.e. N&lt;sub&gt;2&lt;/sub&gt;O and (N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) fluxes. To provoke different levels of oxygen supply and demand, repacked soils from two locations in Germany were incubated in a full factorial design with soil organic matter (1.2 and 4.5 %), aggregate size (2-4 and 4-8mm) and water saturation (70%, 83% and 95% WHC) as factors. The sieved soils were repacked and incubated at constant temperature and moisture and gas emissions (CO&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O) were monitored with gas chromatography. The &lt;sup&gt;15&lt;/sup&gt;N tracer application was used to estimate the N&lt;sub&gt;2&lt;/sub&gt;O reduction to N&lt;sub&gt;2&lt;/sub&gt;. The internal soil structure and air distribution was measured with X-ray computed tomography (X-ray CT).&lt;/p&gt;&lt;p&gt;The interplay of anaerobic soil volume fraction (ansvf) as an abiotic proxy of oxygen supply and CO&lt;sub&gt;2&lt;/sub&gt; emission as a biotic proxy of oxygen demand resulted in 81% and 84% explained variability in N&lt;sub&gt;2&lt;/sub&gt;O and (N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) emissions, respectively. These high values dropped to 5-30% when only ansvf or CO&lt;sub&gt;2&lt;/sub&gt; was considered indicating strong interaction effects. The extent of N&lt;sub&gt;2&lt;/sub&gt;O reduction in combination with ansvf and CO&lt;sub&gt;2&lt;/sub&gt; even increased the explained variability for N&lt;sub&gt;2&lt;/sub&gt;O fluxes to 83%. Average O&lt;sub&gt;2&lt;/sub&gt; concentration measured by microsensors was a very poor predictor due to the extreme variability in O&lt;sub&gt;2&lt;/sub&gt; at short scales in combination with the small footprint of the micro sensors probing only 0.2% of the entire soil volume. The substitution of predictors by independent, readily available proxies for O&lt;sub&gt;2&lt;/sub&gt; supply (diffusivity based on air content) and O&lt;sub&gt;2&lt;/sub&gt; demand (SOM) leads to a reduction in predictive power.&lt;/p&gt;&lt;p&gt;To our knowledge this is the first study analyzing total denitrification in combination with X-ray CT image analysis, which opens up new perspectives to estimate denitrification in soil and also contribute to improving models of N&lt;sub&gt;2&lt;/sub&gt;O fluxes and fertiliser loss at all scales and can help to develop mitigation strategies for N&lt;sub&gt;2&lt;/sub&gt;O fluxes and improve N use efficiency.&lt;/p&gt;