An in situ method for real-time measurement of gas transport in soil

摘要

Soil aeration is central to the biogeochemistry of soil. Gas transport in soil is governed mainly by molecular diffusion and depends on the soil gas diffusion coefficient (D-S). Several methods exist to determine this property based on field and laboratory measurements. These methods, however, are unsuitable for continuous monitoring of D-S over time. Moreover, non-diffusive processes can affect gas transport in soil during certain situations that last several hours only and discontinuous measurements might fail to identify such processes. We developed a novel in situ method for the real-time monitoring of gas transport in soil. Helium (He) was injected continuously into the soil and the resulting steady-state gas profile was monitored. We used a gas sampling probe with very permeable membranes, which enables passive sampling of the soil gas at different depths. The D-S profile was modelled by inverse finite-element modelling (FEM) of an exact geometrical model of the sampler and soil. Molecular diffusion was assumed to be the only process of gas transport. The method was tested in the laboratory with different granular materials and in two field studies. The D-S values obtained with this new method agreed well with reference measurements from soil cores and diffusion models. Soil gas diffusivity was monitored over a few days that included a major rain event. During this period the effect of increasing soil moisture on gas transport in soil could be observed in real time. Our novel method is suitable for monitoring gas transport in soil over several days. This enables the monitoring and investigation of situations including non-diffusive transport processes that result from barometric pressure changes or strong wind events.