Carbon allocation to ectomycorrhizal roots and mycelium colonising different mineral substrates

•  Ectomycorrhizal fungi occur abundantly in the mineral horizons of forest soils, but their interactions with mineral substrates are largely unknown. We have examined the proliferation of ectomycorrhizal roots and mycelium colonising different mineral substrates. •  By exposing the shoots of Pinus sylvestris seedlings to air containing ¹⁴ CO₂ , the carbon allocation patterns in intact ectomycorrhizal associations could be monitored using electronic autoradiography. •  In plants colonised by either Hebeloma crustuliniforme or Piloderma fallax, a larger fraction of the photosynthetically derived carbon was allocated to a mineral soil substrate compared with a Sphagnum peat. In mycorrhizal seedlings colonised by H. crustuliniforme, carbon allocation was significantly greater to roots and mycelia colonising patches of pure potassium feldspar than to those in patches of quartz. •  These results suggest that ectomycorrhizal mycelia may respond to the presence of different mineral substrates by regulating their growth and activity.