Pressure pumping of carbon dioxide from soil

Recent interest in atmospheric increases in carbon dioxide have heightened the need for improved accuracy in measurements of fluxes of carbon dioxide from soils. Diffusional movement has long been considered the dominant process by which trace gases move from the subsurface source to the surface. Although bulk transport resulting from pressure fluctuations has been considered as a possible augmentation to diffusional movement, only within the last 5-10 years has there been sufficient evidence to justify a systematic examination of this possible mechanism. We have measured carbon dioxide fluxes from the soil surface under conditions of natural and artificial pressure pumping at the soil surface. Concurrent measurements of surface fluxes, soil temperature and moisture, field carbon dioxide concentrations at various depths in the soil, wind speed, and high-frequency (1 Hz) pressure fluctuations in the lee of a fence in an agricultural field enable us to calculate diffusional fluxes for comparison with measured fluxes under various pressure pumping regimes. Preliminary results show that pressure fluctuations due to fluctuations in wind speed and wind direction interacting with the fence penetrate, with modest attenuation, to at least 60 cm in bare, nearly dry soil. We also found carbon dioxide fluxes at the surface to be approximately 3 times as large as would be expected from calculations of diffusional flux rates. If these preliminary conclusions withstand further scrutiny, it is likely that surface fluxes of carbon dioxide across the landscape are highly heterogeneous and dependent on landscape features such as trees and orographic features that create local high-frequency fluctuations in static pressure.