Final Activity Report Summary - CONAR (Competition for nutrients between arctic microbes and plants) Arctic ecosystems harbour an enormous stock of carbon in their soils. As these ecosystems are exposed to rapid climate warming with the predicted temperature increases by far exceeding the global estimates, the fate of the soil carbon is of critical importance. If microbial processes accelerate and lead to a loss of carbon from soil to the atmosphere, this is a positive feedback on climate change and leads to further warming. This fellowship focused on responses of soil microbial communities to climate change by applying various state-of-the-art methods to soil samples originating at several long-term field experiments in subarctic Sweden. The main objective of this project was to elucidate how global warming affects interactions within the soil microbial communities, and between plants and rhizospheric microbes in the Arctic.It was found that the microbial communities of an arctic heath had changed in response to seven growing seasons of warming by 1-2 degrees. Now they were limited by a lack of carbon, and they had lower growth rates than the communities from adjacent plots of land without warming treatment. Nevertheless, the warmed soil also lost carbon at higher rates than the control soil suggesting a positive feedback to climatic warming. The changes were probably a result of warming-induced changed in the plant cover and soil organic matter composition. The temperature increase over 7-17 years had not, however, led to adaptation of the microbial communities to the higher temperature. When temperature responses of microbial communities were studied in soil from a climate gradient stretching from the Antarctic Peninsula to the Falkland Islands, it was observed that the microbial communities had adapted to the temperature of their surroundings.Warming does not appear to have clear effects on competition between two microbial groups, fungi and bacteria, for different carbon substrates. However, if warming leads to increased nutrient availability as expected, functioning of the two microbial groups is likely to be affected. A subarctic heath soil that was fertilised for 17 years to simulate increased nutrient availability favoured uptake of different carbon compounds by fungi instead of bacteria. Changes in the competition between fungi and bacteria can cascade to effects on ecosystem food webs and nutrient cycling.