Oxalate secretion by ectomycorrhizal Paxillus involutus is mineral-specific and controls calcium weathering from minerals

Trees and their associated rhizosphere organisms play a major role in mineral weathering drivingcalcium fluxes from the continents to the oceans that ultimately control long-term atmosphericCO2 and climate through the geochemical carbon cycle. Photosynthate allocation to tree roots andtheir mycorrhizal fungi is hypothesized to fuel the active secretion of protons and organic chelatorsthat enhance calcium dissolution at fungal-mineral interfaces. This was tested using 14CO2 suppliedto shoots of Pinus sylvestris ectomycorrhizal with the widespread fungus Paxillus involutus inmonoxenic microcosms, revealing preferential allocation by the fungus of plant photoassimilate toweather grains of limestone and silicates each with a combined calcium and magnesium contentof over 10 wt.%. Hyphae had acidic surfaces and linear accumulation of weathered calcium withsecreted oxalate, increasing significantly in sequence: quartz, granite < basalt, olivine, limestone