Final Activity Report Summary - ERMC (Ericoid mycorrhizas and carbon biogeochemistry in subarctic ecosystems) The main achievements of this project are presented in six scientific papers in which the fellow is the main author in five of them. The soil and plant root environment has been investigated in relation to global change factors, i.e. how factors such as increased temperature and enhanced CO2 level in the atmosphere affect the below-ground carbon cycle. These issues are currently under debate and little is yet know about how the below-ground carbon cycle responds to climate change in arctic and boreal ecosystems.The formation of recalcitrant carbon in soil is an important process controlling recycling of CO2 to the atmosphere. In Olsrud et al. (2010, submitted) we investigated for the first time how the formation of long-term carbon pools in arctic soils, i.e. humic substances, was affected by warming and elevated CO2 concentrations in the atmosphere. We found a significant interaction effect between elevated CO2 and warming on the accumulation of 14C labeled C into humic acids and fulvic acids, as CO2 and warming separately tended to reduce 14C while the combined treatment enhanced 14C incorporation. A significant interaction effect on the C/N ratio in humic acids and fulvic acids also indicated differences in effects of single and combined treatments on the level of decomposition. This emphasises the need of considering elevated CO2 and warming in combination when studying global change effects on the long-term C storage in subarctic ecosystems.The symbiosis between fungi and plants in boreal and arctic ecosystems, i.e. ericoid mycorrhiza as been shown to catalyse decomposition of recalcitrant carbon compounds in soils. In Olsrud et al. (2010) we showed that elevated CO2 concentrations in the atmosphere significantly increased the amount of fungi in roots of ericaceous dwarf shrubs. An increased colonisation level in roots might also result in an increased enzymatic activity in soil affecting carbon decomposition of carbon compounds in soil.Methods used in order to study ericoid mycorrhizal colonisation was developed in Olsrud et al. (2007). Arctic and subarctic dwarf shrub ecosystems are predicted to be exposed to lower light intensities via increased cloudiness in a changed climate, or via shading by canopy-forming trees such as mountain birch, which currently are expanding their distribution towards higher latitudes and altitudes. Such changes in light intensities may tend to reduce the level of ericoid mycorrhizal colonisation and plant organic nitrogen uptake, according to the results presented in Olsrud and Michelsen (2009).In ericoid mycorrhizal symbiosis the fungus obtain most of its carbon from the host plant. However, the amount of carbon transported down to the fungi has never been determined for ericoid mycorrhiza, although this will have major implications for the below-ground carbon cycle in boreal and arctic ecosystems. In Olsrud et al. (in prep.) we show evidence that, for the first time, suggest that approx. 50% of the carbon reaching the plant root is allocated to fungal structures inside roots. Many of the results presented during the course of this project 2007-2010 are of novel character and will be valuable for the scientific community interested in the terrestrial carbon cycle in relation to global change issues.