Turbulence Effect on Gas Transport in Three Contrasting Forest Soils

Abstract

Atmospheric turbulence can affect soil gas transport, and thus violates the assumption that molecular diffusion is the only relevant transport mechanism in the soil. Due to the oscillating character of turbulence-induced pressure pumping the net vertical air flow into/out of the soil is zero. Yet, the dispersion processes induced by the oscillating air movement in the soil pores enhance the gas exchange in the soil, and should be considered when the gradient method is applied. We developed a simple lab method to test the impact of turbulence-driven pressure pumping on soil gas transport using neon as a tracer gas. The experiment was conducted using soil core samples from three contrasting forest sites. The ratio of air permeability and air-filled pore-volume, k a /ε, was found to be suitable to assess the sensitivity of soil gas transport to the pressure-pumping effect. The pressure-pumping effect reached up to 60% of the diffusive flux rates depending on the intensity of pressure pumping. We conclude that the turbulence effect can substantially influence soil gas transport in field studies, especially at sites with a high k a /ε value > 1000 μm 2 . Therefore, we recommend taking this effect into account when soil gas flux is determined using the gradient method. © Soil Science Society of America.