Effects of soil moisture on simulated methane flow under varying levels of soil compaction

Abstract

Soil density plays an important role in regulating the migration of greenhouse gases from terrestrial soils to the atmosphere. Soil moisture is one of the main soil physical controls determining the fate and transport of gases in soils. This study investigated the transport of methane (CH₄) originating from a simulated CH₄ source within a variably compacted pasture soil. Simulations were carried out for dry and variably saturated soils. Steady-state methane flow was simulated as a density-dependent, multiphase flow considering a multicomponent mixture of CH₄, water vapor and air, under different soil moisture conditions. We used measured soil–water characteristic (SWC) and gas diffusivity data at five density levels (1.1, 1.2, 1.3, 1.4, and 1.5 Mg m‾³) to parameterize predictive models. Permeability was estimated using an existing SWC-based saturated hydraulic conductivity function. Results show a distinct effect of soil density on CH₄ concentration profiles within the soil. Clear effects of soil moisture on CH₄ transport could also be seen in differentially compacted soils. Relatively smaller CH₄ concentrations were observed in dry soils where permeability, gas diffusivity, and air-filled porosity were higher. With increasing density, the profile-accumulated concentrations >0.3% increased up to 200 times under the dry condition. In moist soils, on the other hand, smaller air-filled porosity and higher moisture-controlled tortuosity resulted in reduced permeability and gas diffusivity, yielding high CH₄ concentrations in the soil profile with only a maximum fivefold increase in the accumulated concentration with increasing density.