Soil CO2 Emissions: Changes in effective diffusivity due to sustained winds

Soils are the largest terrestrial source of CO2 to the atmosphere and a crucial component in the global carbon balance. Through soil profile and surface monitoring of CO2 concentrations and fluxes it has been shown that soil physical parameters such as moisture and structure can exert strong control over soil CO2 production, storage and emissions. Often, this control is a result of changes in the gas transport potential, or effective diffusivity, of the soil matrix. Variations in wind speeds near the soil surface can induce pressure fluctuations and changes in the structure of the boundary-layer which may lead to changes in effective diffusivity. Sustained winds have the potential to alter both the concentrations of gas in the soil as well as the fluxes of gas to and from the soil matrix. Gas emissions studies at air-water interfaces have demonstrated these effects, showing clear correlations between wind speed and pCO2 but similar research has not been conducted to examine the subsurface effects of wind across the soil-atmosphere interface. A clear understanding of these processes will be critical to understand how they affect flux monitoring strategies as well as to enhance the current understanding of soil CO2 production and emissions. Using fans to generate artificial winds between 3 and 30 km/h and solid state CO2 sensors we examine changes in soil profile CO2 concentrations both in lab conditions using artificial soils with no vegetation and in field conditions under sparse grass cover. Preliminary results show that in both laboratory and field experiments, soil CO2 concentrations are lowered by 10-80% within 20 minutes of the wind speed change. Furthermore, after the fans are shut off, CO2 concentrations recover quickly to their original state implying that there is little change in microbial CO2 production and that the observed CO2 depletion is a result of changes in effective diffusivity. Estimates of the change in effective diffusivity range between 2 and 10 times the original diffusivity of the medium and show a positive, exponential dependence on wind speed. Results from this study will be applicable to measurement and monitoring of soil respired CO2 and to enhancing the understanding of physical constraints on soil C cycling.