Turbulence-driven pressure-pumping - from above the canopy into the soil
Atmospheric Science
Final Report Abstract
Atmospheric gas transport is dominated by flow of air, while gas exchange within soil is dominated by molecular diffusion. During the last years it was shown that strong winds can lead to enhanced soil gas transport. Understanding transport within the soil is highly relevant for soils as sources and sinks of various gases, like oxygen, greenhouse gases, and contaminants like Radon or volatile organic compounds. Nevertheless, there is only very little knowledge about its effect and the involved processes. Wind can have a strong effect on gas transport in snow, in loose materials like gravel, mulch and cracks in soil, and in well-aerated soil, and also laboratory results showed indirectly that wind and pressure fluctuations can have considerable impact on gas transport. Yet, a coherent concept of the coupling between wind, pressure fluctuations and soil gas transport and a direct proof and quantification of the effect is still missing. Thus, our objectives were to measure the effect of wind on soil gas transport directly and to develop a better empirical and conceptional understanding of the mechanisms involved. We studied wind above and within a forest and developed devices and methods that allowed us (a) to measure the tiny pressure fluctuation that are associated with wind, (b) to track the waves of the pressure fluctuations, (c) and to measure soil gas transport directly in the field using Helium as tracer gas. We found that soil gas transport can be enhanced by pressure fluctuation by up to 40% at our study site in the forest. Interestingly, this effect was induced rather by atmospheric pressure fluctuations than by local wind speed near the ground which was always very low. We address the phenomenon as “turbulence-driven pressure-pumping”. 1. Wind results in organized turbulence above the canopy. 3D measurements of air flow above and within the forest canopy showed that typical patterns of airflow like sweeps and ejections occurred at the canopy-atmosphere interface. 2. These turbulence structures induce pressure fluctuations that penetrate into the canopy. Tracking of air pressure fluctuations at the soil surface showed that they propagate in the direction and speed of wind above the canopy, and not in the direction and speed of the local wind. Turbulence structures right above the canopy seem to be the origin of the air pressure fluctuations. 3. Wind-induced pressure fluctuations penetrate into the air-filled soil pore space. Tracking barometric pressure fluctuations with soil depth showed that they are hardly dampened or attenuated but penetrate directly into the well-aerated soil. 4. These pressure fluctuations induce oscillating pumping on the soil pore scale. Theoretical consideration show that changing air pressure will lead to a change in volume and thus a minimally oscillating soil air column within the air-filled soil pores. 5. The oscillating pumping of the soil pore air enhances soil gas transport through dispersion. Laboratory experiments showed that a minimally oscillating pumping can increase gas transport through soil samples by 10-20%.
Publications
- 2016. Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest, Atmosphere, 7: 125
Mohr, M., Laemmel, T., Maier, M., Schindler, D.
(See online at https://doi.org/10.3390/atmos7100125) - 2017. An in situ method for real-time measurement of gas transport in soil. Eur. J. Soil Sci. 68: 156-166
Laemmel, T., Maier, M., Schack-Kirchner, H., Lang, F.
(See online at https://doi.org/10.1111/ejss.12412) - 2017. Direct Observation of Wind- Induced Pressure-Pumping on Gas Transport in Soil. Soil Sci. Soc. Am. J. 81: 770-774
Laemmel, T., Mohr, M., Schack-Kirchner, H., Schindler, D., Maier, M.
(See online at https://doi.org/10.2136/sssaj2017.01.0034n) - 2017. Spatial variability of wind-induced air pressure fluctuations responsible for pressure pumping. Tellus B 69: 1361757
Mohr, M., Laemmel, T., Maier, M., Schindler, D.
(See online at https://doi.org/10.1080/16000889.2017.1361757) - 2018. Effect of local airflow on flux chamber measurements. Geophysical Research Abstracts Vol. 20, EGU2018-15573, EGU General Assembly 2018, Vienna
Laemmel, T., Mohr, M., Maier, M.
- 2019. From above the forest into the soil – how wind affects soil gas transport through air pressure fluctuations. Agr. For. Met. 265, 424-434
Laemmel, T., Mohr, M. Longdoz, B. Schack-Kirchner, H., Lang, F., Schindler, D., Maier, M.
(See online at https://doi.org/10.1016/j.agrformet.2018.11.007) - Vertical air pressure fluctuations in laboratory experiments cannot fully explain the pressure pumping effect on soil gas transport observed in the field. Soil Sc. Soc. Am. J., 2019
Laemmel, T., Mohr, M. Longdoz, B. Schack-Kirchner, H., Schindler, D., Maier, M.
