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Fungal communities are major players in globally important nutrient cycling processes, and form symbioses with most terrestrial plants. In the nitrogen (N) limited Swedish boreal forest, ectomycorrhizal (EcM) fungi colonize most roots of the economically important and stand dominating conifer species, Norway spruce (Picea abies) and Scots pine (Pinus sylvestris), with significant implications for tree nutrition and decomposition processes. Long-term sustainable forestry practices require a deeper understanding of biotic and abiotic factors influencing forest health and tree growth. While high-throughput sequencing technologies such as DNA amplicon sequencing or RNA-based metatranscriptomics have furthered our understanding of fungal communities, there are still many details of EcM symbiosis and decomposition processes that we do not understand. In this thesis, I have used these sequencing methods to further our understanding of fungal communities in the boreal forest, and how they are influenced by forest management and N addition.

In the first part of this thesis, I investigated how early fertilization of seeded or planted seedlings affects seedling growth and survival, and the fungal communities associated with the growing seedlings, assessed by amplicon sequencing. In two manuscripts I show that seeds or seedlings planted into scarified clearcut soil are rapidly colonized by site indigenous fungi, including many EcM species. I show that small doses of added N increase survival of sown seeds and that organic N (in the form of arginine) can increase early root growth of planted seedlings. This light fertilization did not perturb early fungal …