Physical, chemical and biological controls of gas fluxes at the soil-atmosphere interface

Mastering gas fluxes from the soil to the atmosphere is important for several high-stakes societal issues. Measuring and extrapolating these fluxes is a complex exercise due to their spatial and temporal variability. This variability is related to the many, often intertwined, processes that control the transport of gases in soils and at the soil-atmosphere interface. An innovative device has been developed within an experimental platform to study gas fluxes at the surface of a soil column placed under controlled conditions, with long-term, high-resolution monitoring of many parameters. The physical, chemical and biological mechanisms responsible for the variations of gas flux at the soil-atmosphere interface can thus be studied separately. This study focused in particular on the effects of plant metabolism (evapotranspiration, respiration and photosynthesis) as well as water content and barometric pressure. These mechanisms affect the pressure gradient that controls advective gas transport. A constant gas flow at the base of a soil can thus show significant transient variations on time scales ranging from several hours to several days. Numerical modelling has been initiated although no code is currently able to account for two-phase transport in the presence of sharp air/water fronts and evaporation. The numerous experimental results will be used to validate the necessary developments. (author)