From above the forest into the soil – How wind affects soil gas transport through air pressure fluctuations

Abstract

Molecular diffusion is commonly assumed as main physical process of gas transport in soils. However, non-diffusive gas transport processes like the so-called pressure-pumping effect can affect soil gas transport significantly. The pressure-pumping effect has only been detected indirectly and the underlying mechanisms remain unclear. Using a novel in situ method the soil gas transport at a conifer forest site was monitored over a seven-week period. Airflow and air pressure were simultaneously measured above and below the forest canopy and air pressure was also measured in the soil. During episodes of high above-canopy wind speed, the effective soil gas diffusivity temporarily increased due to pressure-pumping. The enhancement of the gas transport rate in the topsoil reached up to 30%. We found that the best meteorological proxy explaining this effect was related to air pressure fluctuations measured at soil surface and not the mean wind speed directly above ground. While sub-canopy wind speeds continuously decreased from the bottom of the tree crown to the soil surface, amplitudes of the air pressure fluctuations were nearly constant in the whole sub-canopy profile and in the soil. We hypothesize that the air pressure fluctuations responsible for pressure-pumping are related to characteristics of above-canopy airflow rather than to airflow directly above the soil surface.