A significant portion of plant photosynthate enters the soil through root exudation of easily decomposable carbohydrates, and such inputs are known to respond to changes in environmental conditions and resource availability. What is less known is the role of mycorrhizal fungi on mediating this flux. Elevated atmospheric CO₂ and N deposition affect the community composition of mycorrhizal fungi, but the influence of such shifts in taxa on the quantity of carbon loss is largely unknown. We investigated the indirect influence of elevated CO₂ and N fertilization on root and mycelial exudation as mediated by shifts in ectomycorrhizal (ECM) community composition. In a greenhouse study, we germinated loblolly pine (Pinus taeda L.) seeds in intact (i.e. undisturbed) soil cores collected from the Duke Forest FACE site, NC, where loblolly pine stands have been exposed to elevated CO₂ for 14 years and N fertilization for five years. Seedlings were germinated in soils collected from all 16 plots at the site (two CO₂ levels x two N levels x four replicates) in order to exploit heterogeneity in ECM propagules among the treatments. After 14 weeks of germination, we identified fungal taxa using the DNA sequence similarity of the internal transcribed spacer ribosomal RNA gene region and examined the influence of ECM community composition on root vitality, total non-structural carbohydrates, and exudation rates.

Results/Conclusions

The natural infection of pine seedlings with inocula (spores, mycelia and mycorrhizal roots) from intact forest soil resulted in symbioses with a number of different mycorrhizal species. This presentation will describe (1) how the maximization of the photosynthetic carbon gain affects the carbon loss by root and mycelial exudation through a change in the mycorrhizal community structure and (2) the consequence of a change in the mycorrhizal community structure for the carbon storage in its host tree.