Ectomycorrhizal fungi and past high CO2 atmospheres enhance mineral weathering through increased below-ground carbon-energy fluxes

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.