THE INFLUENCE OF SOIL-GAS TRANSPORT-PROPERTIES ON METHANE OXIDATION IN A SELECTION OF NORTHERN EUROPEAN SOILS

The oxidation of atmospheric methane in soils was measured in situ at a selection of sites in northern Europe, mainly under forest but also under moorland and agricultural arable land and grassland. Our objecti ve was to examine how land use, soil type, and location affected metha ne oxidation through their impact on gas diffusivity and air permeabil ity. Gas diffusivity at the soil surface and, in some cases, after rem oval of any surface organic layer was measured in situ using Freon-22 tracer in a portable probe. For about half of the sites, gas diffusivi ty was also measured in intact topsoil core samples in the laboratory using krypton 85. Air permeability and porosity were also measured on these cores. Although the method of measurement of CH4 oxidation varie d between sites, the same techniques were used to measure soil physica l properties at all sites. CH4 oxidation rates ranged from 0 to 2.5 mg m(-2) d. Diffusivity also covered a very wide range, being lowest in loam cores from wet grassland in Norway and highest in relatively dry, sandy soils in Denmark and Scotland. CH4 oxidation tended to increase with gas diffusivity measured in situ at the soil surface, though the relationship was poor at high diffusivities, presumably because CH4 o xidation was not limited by diffusion. Removal of the surface organic layer reduced in situ diffusivity at the surface and improved its rela tionship with CH4 oxidation rate. Sites where soils had well-developed structure and a loose and permeable organic layer at the surface tend ed to have the highest CH4 oxidation rates. Core measurements, particu larly of air permeability, could not be obtained at some sites owing t o the inability to take suitable samples. Diffusivity measured in core s generally decreased with increasing depth of sampling in the topsoil , with the 50- to 100-mm depth giving the best correlation with CH4 up take; cores from within this layer also gave the highest CH4 oxidation during laboratory incubation. Effective comparisons between sites wer e hampered by the differing responses of CH4 oxidation and diffusivity to soil properties. However, multivariate cluster analysis that inclu ded the above transport variables plus others relevant to CH4 oxidatio n (namely, soil texture; bulk density; air-filled porosity; pH; carbon , nitrogen, and water contents; presence and depth of organic layers; and N deposition) confirmed the importance of soil water content, stru cture and texture in distinguishing different soil and site conditions .