The influence of gas transport and porosity on methane oxidation in soils

Two porosity-dependent parameters that affect gas transport in soils, gas diffusivity and air permeability, were assessed as possible indicators of methane oxidation rates. Soil gas diffusivity was measured in intact cores in the laboratory and in situ, and air permeability was measured in intact cores. An in situ method of measuring gas diffusivity was specifically modified for this purpose to use Freon in a portable probe. A laboratory (core) method of measuring gas diffusivity, using krypton 85, was also employed. Measurements were made at the soil surface and at a range of depths within the topsoil, in conjunction with in situ measurements of CH4 oxidation, in forest, arable, and set-aside soils at a lowland site and in a forest soil at an upland site, in southeast Scotland. The surface layer of soil caused marked variations in diffusivity measurements, particularly with the in situ method. At the upland site, where a 50-mm-thick surface organic layer was present, the in situ technique revealed a sharp decrease in diffusivity with depth. Only where gas transport was low, in the set-aside soil, was methane oxidation rate influenced by gas transport changes associated with increasing soil water content. Differences in CH4 oxidation rates related better to core gas diffusivities than to in situ diffusivities. The relationship was best at 50-150 mm depth where diffusivities were lower than at the surface. Air permeability, which is affected more by soil structure than diffusivity, appeared to be as relevant as the latter parameter to CH( )oxidation rate, particularly for land use changes associated with agriculture. Thus CH4 oxidation rate appears to be influenced by gas transport properties and soil structure, either at the surface in the litter layer or below the surface where the oxidizing microorganisms are likely to occur.