Methane (CH4) is one of the most potent greenhouse gases and its increasing concentration in the Earth’s atmosphere is linked to today’s global warming. The types of land and land-use have an impact on net CH4 fluxes, e.g. wetlands are generally net CH4 emitters while upland forest soils are a sink for CH4. This project aimed to elucidate the effect of afforestation and reforestation on net CH4 fluxes and to determine the control of the CH4-oxidising bacteria (methanotrophs) on net CH4 flux rate. This was investigated using a combination of molecular (T-RFLP, cloning/sequencing, microarray) and activity-specific (PLFA-SIP) approaches. Several sites were selected to analyse soil methanotrophs under shrubs regenerating after a fire compared to a native mature forest (in New Zealand), and under bog, grass, heath, pine and birch vegetation (in Scotland). Furthermore, a simple bottom-up approach was applied to seasonal measurements of local net CH4 fluxes in Scotland. These were upscaled to annual values in order to estimate the contribution to the national CH4 budget for each habitat investigated. The effect on CH4 mitigation of the conversion of different types of non-forested habitat to forests was then estimated. Afforestation/reforestation was always found to induce net CH4 oxidation at rates much faster than previously estimated. This preliminary analysis suggests that heathland conversion to birch forest was beneficial in term of CH4 sinks but it also induced large and permanent losses of soil C. However, bog afforestation with pine trees can potentially neutralise the national CH4 emissions from non-forested areas, while preserving soil C stocks. This project also revealed that changes in net CH4 flux due to land-use changes were closely related to shifts in the structure of the methanotrophic community. The relative abundance of members of the USCα cluster (high-affinity methanotrophs) was a strong predictor of net CH4 fluxes. Finally, the sole presence of trees suggested a niche-specific adaptation of the methanotrophs, which may have been correlated to some of the soil characteristics.