Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest

Abstract

High-precision differential air pressure measurements were conducted in the below-canopy space of a Scots pine forest and in the forest soil to investigate small air pressure fluctuations and their effect on soil gas flux. In addition to air pressure measurements, tracer gas concentration in the soil and airflow characteristics above and below the canopy were measured. Results suggest that air pressure fluctuations in the frequency range of 0.01 Hz-0.1 Hz are strongly dependent on abovecanopy wind speed. While amplitudes of the observed air pressure fluctuations (<10 Pa) increase significantly with increasing above-canopy wind speed, the periods decrease significantly with increasing above-canopy wind speed. These air pressure fluctuations are associated with the pressure-pumping effect in the soil. A pressure-pumping coefficient was defined, which describes the strength of the pressure-pumping effect. During the measurement period, pressure-pumping coefficients up to 0.44 Pa·s −1 were found. The dependence of the pressure-pumping coefficient on mean above-canopy wind speed can be described well with a polynomial fit of second degree. The knowledge of this relation simplifies the quantification of the pressure-pumping effect in a Scots pine forest considerably, since only the mean above-canopy wind speed has to be measured. In addition, empirical modeling revealed that the pressure-pumping coefficient explains the largest fraction of the variance of tracer gas concentration in the topsoil.

1D Air Pressure Fluctuations Cannot Fully Explain the Natural Pressure‐Pumping Effect on Soil Gas Transport

“…The other samples (I‐VII) were subjected to air pressure fluctuations of only A = 80 Pa (peak‐to‐peak value) and f = 0.1 Hz ( T = 10 s), corresponding to PPC = 16 Pa s −1 . These A and f values are the highest values for air pressure fluctuations observed during previous field measurements (Mohr et al, 2016).…”

“…Air pressure was measured inside and outside the chamber using two differential air pressure sensors (GMSD 2.5 MR, Greisinger Electronic GmbH, Regenstauf, Germany). One outlet of each sensor was connected to a reference volume (Mohr et al, 2016).…”

“…To study soil gas transport through air pressure fluctuations, soil air permeability k a is an important parameter (Fukuda, 1955) and commonly observed values are lower than 2000 μm 2 (Maier et al, 2012). Pressure‐pumping effect seems to depend on the amplitude ( A ) and period ( T ) of the air pressure fluctuations; A of several tenths of pascals (in the following, A will always refer to the peak‐to‐peak value) and T of several tens of seconds, i.e., frequencies ( f ) between 0.01 Hz and 0.1 Hz, have been observed during field measurements (Mohr et al, 2016).…”

“…The pumping‐induced oscillation of the soil air column leads to the ejection of tiny air parcels at the soil‐atmosphere interface, which are then taken away by lateral movement of atmospheric air, similar to the effect of barometric pumping. However, the amplitude of the gas displacement due to this oscillation is in the sub millimeter to millimeter scale in real soils (Farrell et al, 1966) for real air pressure fluctuations (Mohr et al, 2016), so that this oscillation cannot fully explain the observed pressure‐pumping effect. Scotter et al (1967) first addressed the PPE as a dispersion effect.…”

Effect of near‐surface wind speed and gustiness on horizontal and vertical porous medium gas transport and gas exchange with the atmosphere

“…Although several experimental studies of the effect of wind on gas exchange have been carried out (Ishihara et al ., ; Poulsen & Sharma, ; Maier et al ., ; Ganot et al ., ; Mohr et al ., ), only a few have involved experimental assessment of wind‐induced subsurface gas transport in more than one dimension under both steady and gusty wind conditions. Nachshron et al .…”