Project Details
Projekt
Turbulence-driven pressure-pumping - from above the canopy into the soil
Subject Area
Soil Sciences
Atmospheric Science
Atmospheric Science
Term
from 2013 to 2019
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 243975681
Gas transport in the atmosphere is dominated by turbulent exchange and laminar airflow. In contrast, in the soil mass-flow of air is negligible; instead soil gas transport is governed by molecular diffusion. At first glance, the transport processes in the atmosphere and in the soil appear not to be linked with each other. However, it was observed in previous field studies that soil gas transport rates can be increased by several 10% during periods of strong wind. This increase is due to turbulence-induced pressure fluctuations that propagate into the air-filled soil pores. These pressure fluctuations cause a minimally oscillating airflow in the soil pores (Pressure-pumping effect). The direct contribution of the airflow to the overall gas exchange is negligible due to the alternating direction of the flow. Yet, the movement of air is accompanied by dispersion that leads to an enhanced gas transport against the underlying concentration gradient. Neglecting the pressure-pumping (PP) effect can result in large errors when the gradient method or chambers are used to estimate gas fluxes from or within the soil. This uncertainty represents a problem especially for long term monitoring of greenhouse gas fluxes.We hypothesize:(H1) The PP-effect is dependent on soil characteristics.(H2) Amplitudes and frequencies of surface pressure fluctuations are affected by canopy characteristics.(H3) The chamber measurements of gas fluxes are affected by surface pressure fluctuations and airflow.(H4) CH4 consumption of upland forest sites is enhanced by the PP-effect.The hypotheses 1, 3 and 4 will be tested by laboratory experiments using samples of different soils and soil moisture regimes. Hypothesis 2 will be tested by conducting field studies at different measurement sites.Objectives of the study are: (O1) developing models to assess the effect of soil structure on the PP-effect, (O2) assessing the effects of canopy roughness on pressure fluctuations, (O3) defining thresholds to identify sites where the PP-effect is highly relevant, (O4) developing correction factors for chamber methods, (O5) developing correction factors for the gradient method, (O6) investigating how CH4 uptake in upland forest soils is affected by the PP-effect.A deeper insight into the previously neglected PP-effect will help significantly increasing the reliability and precision of measurements of soil gas fluxes, which represent the methodological base for a large field of ecological research.
DFG Programme
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