ECTOARC · Ectomycorrhizal fungi and the stability of soil organic matter in a changing Arctic

Ectomycorrhizal fungi and the stability of soil organic matter in a changing Arctic

The Arctic region is currently facing large changes in climatic conditions, with expected consequences for the vulnerable terrestrial ecosystems. Northern ecosystems contain a large proportion of the global soil carbon (C) stock, and there is a risk that environmental changes will lead to a release of carbon dioxide from this pool, substantially amplifying atmospheric warming potential. However, populations of plants and microorganisms will likely change with changed environmental conditions. An increased abundance of shrubs and trees forming ectomycorrhizal symbiosis with soil fungi has already been observed across the Arctic tundra biome as a response to recent warming, and this trend is expected to continue. Ectomycorrhizal mycelia may contribute significantly to organic matter formation but also contribute to the degradation of organic matter by producing extracellular enzymes. This two-year Marie Curie project has explored the possible role of ectomycorrhizal fungi as main drivers of soil C dynamics when tree growth increase in northern biomes. The project explored two natural gradient systems in northern Sweden, a subarctic-alpine forest-heath ecotone and a long-term boreal forest chronosequence, in order to characterise shifts in fungal communities in relation to important ecosystem functions. The project combined novel molecular tools, high-throughput pyrosequencing, to characterise fungal communities, with ecosystem level measurements, such as C, nutrient and stable isotope pools and biomass estimates. Results from the forest-heath ecotone point to a strong, positive coupling between tree abundance, mycorrhizal fungal growth and decomposition rate of humus. Ectomycorrhizal mycelial production was ten-fold higher in forest than in the heath. In contrast, the total soil C pool in the forest was one-third of the pool in the health systems, although litter production was larger here. This points to a much higher turnover rate of soil organic matter in the forest, possibly related to higher ectomycorrhizal activity. The dataset of 80 000 fungal sequences also clearly showed that fungal communities shifted - both along the ecotone and down soil profiles. However, an overwhelming number of unknown taxa (over 90 % of the 1604 taxa) were found, highlighting a need for further research in the area. In conclusion, this study strongly suggests that trees, through their interaction with ectomycorrhizal fungi, play a more direct role in soil C turnover than previously thought. Two manuscripts are in preparation from this work; one concerning the bioinformatics pipeline that we developed to process the sequence data (see http://www.scata.mykopat.slu.se online) and one on results from the field study. This Marie Curie fellowship has led to a continuation contract between the fellow and Department of Forest Mycology and Pathology, and results from two other projects will be available within the coming year. If the direct coupling between tree production and humus decomposition via ectomycorrhizal fungal activities suggested by this project is confirmed by further studies, it will be extremely important to include this in models predicting future C balance of northern regions. Better predictions of climate effects will be of great value to people dependent in whole or in part on tundra resources to maintain their livelihoods, managers of wildlife and natural resources in far northern regions, and to public policymakers and others with interests in tundra regions. Arctic and Boreal ecosystems represent an important pool in both national and global C budgets. Increased certainty in predictions of northern ecosystem responses to global change, thus, is pivotal for the planning of sustainable management of northern natural resources and for sustainable development of the surrounding society that depend on them. Contact details: K. E. Clemmensen, Uppsala Biocenter, SLU, Department of Forest Mycology and Pathology, Karina.Clemmensen@mykopat.slu.se

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