Gas fluxes from soil to atmosphere: what do the eddies?

Thomas Laemmel1, Martin Maier1, Manuel Mohr2, Dirk Schindler2, Friederike Lang1, Helmer Schack-Kirchner1
1 Chair of Soil Ecology, Albert-Ludwigs-University Freiburg
2 Chair of Meteorology, Albert-Ludwigs-University Freiburg

P 3.8 in Fluxes between the atmosphere and ecosystems

Poster Session 2 on Tuesday, 16:30-18:00

Gas transport in soil is mainly governed by molecular diffusion and is usually modelled using the first Fick's law. However, mass flow of air due to pressure differences can play a significant role in gas transport.

Soil can be considered as a „high-filter“ for pressure waves i.e. only pressure waves with low frequencies (day-1 to h-1) can deeply propagate into it and cause advective transport. Usually, high-frequency pressure fluctuations with frequencies > 1Hz are assumed to be totally attenuated in the first centimeters of the soil. However some previous works showed that pressure fluctuations with frequencies 0.01 Hz – 1Hz can increase gas fluxes from soil to the atmosphere - especially for well-aerated soils – by inducing dispersion in the air-filled pores. This phenomenon is often called turbulence-driven pressure-pumping (because it is caused by eddies rolling out at soil surface). Yet, quantification of turbulence-driven pressure pumping is challenging and many details are unknown because of methodological problems.

Aim of our project is the quantification of turbulence driven gas transport. Continuous field monitoring campaigns will be conducted to better understand this process and acquire data to model it. We will feed in continuously helium as tracer gas (non-reactive and high diffusion coefficient in air) into the soil and analyze the resulting concentration profile in the overlying soil using a Micro GC module. Based on this approach, we will inversely model the gas transport around our set-up and determine soil transport parameters. We assume that dispersion phenomena are negligible during calm meteorological conditions. Thus we will determine diffusion parameters under constant pressure conditions, while the determination of transport parameters during windy conditions will enable to evaluate the contribution of the turbulence-driven pressure pumping to overall gas transport. Our results will enable the assessment of uncertainties related to gas flux estimates neglecting turbulence driven pressure pumping.

Letzte Änderung 19.06.2014