The concentration of CO₂ in the atmosphere is increasing as a result of human activities, and elevated CO₂ stimulates carbon cycling in many ecosystems. Mycorrhizal fungi receive a direct supply of carbohydrates derived from the photoassimilates of their host plants and translocate carbon through the mycelia into the surrounding soil, thereby influencing carbon release from plants to soil. Given the central role of symbiotic fungi in carbon and nutrient cycles, they are potentially an important component influencing how ecosystems respond to elevated CO₂. Understanding how mycorrhizal fungal communities respond to elevated CO₂, and the mechanisms behind the response, is therefore highly relevant to predicting the effects of future climate change.

Using a combination of different methods, e.g. Fourier Transform Infra-Red spectroscopy (FTIR), stable isotope probing (SIP) and molecular tools, we have studied the effects of elevated CO₂ on the plant-fungus-soil system. The following questions were addressed: 1) How does increasing carbon availability effect the chemical composition of mycelium?, 2) Will increased carbon availability alter exudation patterns not only quantitatively but also qualitatively?, and 3), What are the effects of this carbon release on soil microbial communities?

Results/Conclusions

Our preliminary results from the FTIR study showed both inter- and intra-specific differences in chemical composition of ectomycorrhizal mycelia when grown at different carbon availability. For example, the species Hebeloma velutipes was not affected, while the polysaccharide, lipid and protein content was altered for Suillus bovinus depending on carbon availability. For exudation, we showed a clear impact of elevated CO₂ on the production of soluble low molecular weight organic compounds and dissolved organic carbon, and that exudates increased by 120-270% with increased carbon availability. Exudation of organic acids was negatively affected by inorganic nitrogen additions and decreased by 30-85% compared to the organic nitrogen treatment, irrespective of CO₂ treatment. Further, the results suggest that specific exudation rates increased, and that individual species respond differently. The release of exudates in soil microcosm systems caused altered bacterial community structure under elevated CO₂ conditions, and was dependent on fungal species. The results show marked differences between individual fungal species in their response to changed carbon availability, both in chemical composition and exudation.This may feed back on the function of mycorrhizal systems under changing environmental conditions, as community composition changes.