Mycorrhizal fungi are obligate symbionts of most plants and receive fixed carbon (C) from their hosts in exchange for nutrients they supply to the plants. However, the amount of C allocation and rate of C flux to mycorrhizal fungi have been difficult to assess, preventing detailed understanding of the importance of current photosynthate supply in mycorrhizal ecophysiology and their role in carbon cycling in terrestrial ecosystems. During the phased shutdown of the AspenFACE experiment in Rhinelander there was a unique opportunity to study the carbon flux to mycorrhizal fungi through use of stable C isotope ratios. At this field experiment plots receiving elevated CO2 were supplied with CO2 depleted in 13C compared to ambient air, giving us the opportunity to follow the ambient C isotope signal to various ecosystem pools after CO2 shutdown. Sporocarps, foliage and soil samples from all plots were collected before, and at several intervals after CO2 shutdown and analyzed for C isotopes.
Within the first 5 days after CO2 shutdown a significant change in δ13C was observed in sporocarps of most ectomycorrhizal fungal (EMF) species. Isotopic 13C values converged with those in control plots about 20 days or more after CO2 shutdown. The observed lag in δ13C change of EMF sporocarps indicates that EMF derive part of their fixed C from short-term host or fungal storage pools of carbohydrates rather than receiving all directly from current photosynthate. This storage pool likely buffers mycorrhizal fungi against short-term fluctuations in C fixation and belowground flux rates.