Does carbon partitioning in ectomycorrhizal pine seedlings under elevated CO2 vary with fungal species?

Enhanced soil respiration in response to elevated atmospheric CO2 has been demonstrated, and ectomycorrhizal (ECM) fungi are of particular interest since they partition host-derived photoassimilates belowground. Although a strong response of ECM fungi to elevated CO2 has been shown, little is still known about the functional diversity among species. We studied carbon (C) partitioning in mycorrhizal Scots pine seedlings in response to short-term CO2 enrichment, using seven ECM species with different ecological strategies. Mycorrhizal associations were synthesised and seedlings grown in large Petri dishes containing peat: vermiculite and nutrient solution for 10-15 weeks, after which half of the microcosms were exposed to elevated CO2 treatment (710 ppm) for 15 days and the other half were kept in ambient CO2 treatment. Partitioning of C was quantified by pulse labelling the seedlings with (CO2)-C-14 and examining the distribution of labelled assimilates in shoot, root and extraradical mycelial compartments by destructive harvest and liquid scintillation counting. Fungal biomass was determined with PLFA analysis. The respiratory loss of (CO2)-C-14 was on average greater in the elevated CO2 treatment for most species compared to the ambient CO2 treatment. More label was retrieved in the shoots in the ambient CO2 treatment compared to elevated CO2 (significant for P. involutus and P. fallax). Greater amounts of label were found in the extraradical mycelial compartment in all species (except P. involutus) in elevated CO2 compared to ambient CO2 (significant for L. bicolor, P. byssinum, P. fallax and R. roseolus). Fungal biomass production increased significantly with elevated CO2 for two species (H. velutipes and A. muscaria); three species (P. fallax, P. involutus and R. roseolus) showed a similar but non-significant trend, whereas L. bicolor and P. byssinum produced less biomass in elevated CO2 compared to ambient CO2. When C-14 in the mycelial compartment and respiration was expressed per unit fungal PLFA the difference between CO2 treatments disappeared. We demonstrated that different ECM fungal isolates respond differently in C partitioning in response to CO2 enrichment. These results suggest that under certain growth conditions, when nutrients are not limiting, ECM fungi respond rapidly to increasing C-availability through changed biomass production and respiration.