The influence of afforestation and tree species on soil methane fluxes from shallow organic soils at the UK Gisburn Forest Experiment

There is growing evidence that land use is an important factor in influencing soil methane (CH4) fluxes, and afforestation is viewed as a potential tool for mitigating CH4 releases from soils. However, the influence of different tree species on soil CH4 fluxes is not well understood. We measured soil CH4 fluxes under four tree species and grassland on similar soils at the Gisburn Experimental Forest (NW England) at 2 weekly intervals over 12 months using a static chamber technique. The treatments were Norway spruce (Picea abies), Scots pine (Pinus sylvestris), oak (Quercus petraea), alder (Alnus glutinosa) and grassland. Positive soil CH4 fluxes were observed from grassland plots (average 4.6 kg/ha/year) and negative fluxes from all four tree species (average of all tree species -0.5 kg/ha/year). There were, however, no statistically significant differences between individual treatments. Soil water table depth and moisture content had the greatest influence on soil CH4 fluxes. It is possible that the afforestation of shallow organic and/or poorly drained soils such as these may have a relatively low capacity for mitigating CH4 fluxes. Although methanotrophic bacteria may exist (i.e. there is the potential for oxidation), they may not be able to dominate due to their requirements for specific environmental conditions.