Using X-ray Computed Tomography to Describe the Dynamics of Nitrous Oxide Emissions during Soil Drying

International audienceWater in soil is known to be a key factor for controlling N2O emissionsbecause N2O is mainly produced by denitrification in anoxic environments.In this study, we proposed a methodology to image the water and soil structureof a soil sample with X-ray computed tomography while controlling thehydric state and monitoring N2O fluxes. We used a multistep outflow systemto apply two wetting–drying cycles to an undisturbed soil. The soil core wasscanned with coarse-resolution X-ray computed tomography, one time duringwetting and several times during drying, to measure quantitative andqualitative indicators of the pore network. Nitrous oxide emissions werehigher during the first (C1) than during the second (C2) wetting–drying cyclefor both the wetting and the drying phases. Fluxes increased quickly afterthe beginning of the drying phase to reach a peak after 5 h. Differences inthe intensity of N2O emissions between the two cycles were attributed todifferences in the water saturation, air-phase connectivity, and relative gasdiffusion coefficient, which led to more or less N2O production, consumption,and entrapment in the soil. The speed of the N2O emissions at the beginningof the drying phase depended on the rate of increase of the air-filledpore volume and connectivity, and was especially well described by theestimated relative gas diffusion coefficient. Parameters of the soil structurewere not able to explain completely the intensity of N2O emissions duringdrying; N2O production and consumption factors were also involved