Field studies indicate an intensification of mineral weathering with advancement from arbuscular mycorrhizal (AM) to later-evolving ectomycorrhizal (EM) fungal partners of gymnosperm and angiosperm trees. We test the hypothesis that this intensification is driven by increasing photosynthate carbon allocation to mycorrhizal mycelial networks using 14CO2-tracer experiments with representative tree–fungus mycorrhizal partnerships. Trees were grown in either a simulated past CO2 atmosphere (1500 ppm)—under which EM fungi evolved—or near-current CO2 (450 ppm). We report a direct linkage between photosynthate-energy fluxes from trees to EM and AM mycorrhizal mycelium and rates of calcium silicate weathering. Calcium dissolution rates halved for both AM and EM trees as CO2 fell from 1500 to 450 ppm, but silicate weathering by AM trees at high CO2 approached rates for EM trees at near-current CO2. Our findings provide mechanistic insights into the involvement of EM-associating forest trees in strengthening biological feedbacks on the geochemical carbon cycle that regulate atmospheric CO2 over millions of years.