Ecosystems all over the world are affected by climate change, and especially so high latitude and altitude ecosystems. One consequence of global warming in Norway is an upward shift in the mountain birch forest line. A large fraction of earths terrestrial carbon (C) are stored in the soils of these northern ecosystems. As the soil C dynamics primarily is regulated by belowground microorganisms, a better understanding of these communities in a changing climate is crucial. The aim of this MSc thesis was to analyze compositional chances in the soil biota across the mountain birch forest line ecotone, with a main emphasis on fungi, as well as their environmental drivers. The results are seen in light of soil C sequestration processes. From nine different sites, soil samples were obtained along 200 m transects stretching from subalpine mountain birch forest to low-alpine vegetation. Compositional changes in the belowground biota were analyzed using DNA metabarcoding of the rDNA 18S and ITS2 markers, while ergosterol was used to quantify changes in fungal biomass. Regardless of site, there was a strong gradient in community composition across the ecotone, acknowledging the ecotone as the primary structuring gradient shaping the biota in soil. There were relative higher abundance of basidiomycetes and mucoromycetes, including ectomycorrhizal and saprotrophic fungi, below the forest line. On the contrary, there were relatively higher abundance of ascomycetes, including archaeorhizomycetes, ericoid mycorrhizal fungi and dark septate endophytes above the forest line. Further, soil C and ergosterol showed a strong increase towards the low-alpine vegetation. The predominance of ectomycorrhizal and saprotrophic fungi below the forest line may contribute to enhanced C turnover, while the predominance of root-associated ascomycetes above the forest line, of which many have melanized mycelia, may promote a slower C turnover and increased C sequestration here. If the rise in forest lines continues, a corresponding shift in fungal communities will take place. Based on the results from this thesis, this will most likely lead to loss of soil C, which ultimately may affect the global C cycle.