Pathways for below-ground carbon transfer between paper birch and Douglas-fir seedlings

Background: Carbon can move below ground between ectomycorrhizal plants, but the relative importance of transfer through common mycorrhizal networks (CMNs) or soil pathways remains unclear. We studied carbon transfer between paper birch (Betula papyrifera) and Douglas-fir (Pseudotsuga menziesii) seedlings grown in adjacent root-restrictive pouches inside root chambers. Aims: The objective of this study was to compare transfer between CMN pathways and soil pathways by testing if: (1) carbon transfer between paper birch and Douglas-fir is bi-directional, (2) there is a net gain in carbon by one of the tree species; and (3) more carbon is transferred through the CMN pathway than the soil pathway. Methods: Following 8 months in the greenhouse, hyphal linkages crossing root pouches were either severed or left intact. Seedlings were then reciprocally labelled with (CO2)-C-13 and (CO2)-C-14. Results: We found carbon was transferred bi-directionally, with a 2-3% net gain in carbon by Douglas-fir from paper birch. Both bi-directional and net transfer occurred where the CMN was severed, but transfer was greater where it was left intact, indicating that carbon was transferred through both the soil and CMN pathways. Conclusions: Our results suggest that significant amounts of labelled carbon were transferred between plant species, showing for the first time in a balanced pulse-labelling experiment that approximately three times as much carbon was transferred through CMN pathways than soil pathways. That most carbon transfer occurred through CMNs could affect competitive interactions between establishing seedlings, favouring some included in the CMN while disadvantaging others, thus fundamentally altering our understanding of how interspecific interactions alter community structure.