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The role of ectomycorrhizal fungi in carbon transfer within common mycorrhizal networks Philip, Leanne Jane


Ectomycorrhizae, one of the largest groups of mycorrhizal fungi, have low host specificity, colonizing mainly tree species in temperate forest systems worldwide. Because these fungi readily colonize many trees, hyphae form belowground connections between root systems of the same or different tree species whereby carbon and nutrients flow directly between rhizospheres. This biological phenomenon has been described as a consequence of common mycorrhizal networks and has been suggested as one potential factor in regulating composition and structure of plant communities. Ectomycorrhizae and belowground carbon transfer between paper birch (Betula papyrifera Marsh.) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) was studied in three laboratory studies and one field study to determine the role ectomycorrhizal fungi play in belowground carbon transfer between neighbouring mycorrhizal plants. Pot systems, root chambers, and pulse labeling [i.e labelling] were used in the laboratory to examine: (1) minimum detection levels required to detect belowground carbon transfer, (2) the magnitude and direction of carbon transfer through hyphal pathways compared to alternate soil pathways, and (3) the effects of fungal species on the magnitude of carbon transfer. Results from the laboratory show that >16 ml ¹³CO₂ and at least 10 μCi ¹⁴CO₂ pulse-labels are required to detect carbon transfer and is size dependent, that more net carbon transfer occurs when hyphal connections are left intact than not, and that fungal specificity tends to affect carbon transfer. Reciprocal isotopic labeling of paper birch and Douglas-fir in spring, summer and fall was used to determine phenological effects on the magnitude and direction of transfer. Carbon moved from paper birch to Douglas-fir in summer and in the opposite direction in spring and fall representing 4-34% net transfer (proportion of the total isotope assimilated in the system) through hyphal pathways. Phenology also affected carbon transfer through soil pathways. These laboratory and field findings support and extend previous studies in belowground carbon transfer, providing strong evidence for the role ectomycorrhizal fungi play in belowground transfer. Further studies are required to determine if transfer amounts observed in these experiments are of a sufficient magnitude to influence plant biodiversity, production, and fitness.

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