Intensification of Soil Mineral Weathering through Mycorrhiza Evolution: From Liverworts to Forests, Pedogenesis to Global Biogeochemical Cycles.

The co-evolution mycorrhizal fungi with liverwort-like land plants over 440 million years ago set in chain processes that led to the greening of the continents by plants of increasing biomass, rooting depth, nutrient demand and capacity to alter soil minerals, culminating in modern forested ecosystems.  We present experimental and observational evidence in support of four linked hypotheses:  (1) that high atmospheric CO₂ concentrations of the early Palaeozoic strongly favoured the establishment of mycorrhiza-like associations in liverworts, to increase their nutrient uptake; (2) as plants evolved in stature, biomass, and rooting depth, their mycorrhizal fungal partnerships received increasing amounts of plant photosynthate,  (3) this enabled intensification of plant-driven fungal weathering of rocks to release growth-limiting nutrients such as the element P from calcium phosphates; (4) in turn, this increased land-to-ocean export of Ca and P and enhanced ocean carbonate precipitation impacting the global carbon cycle and biosphere-geosphere-ocean-atmosphere interactions over the past 410 Ma. 

These hypotheses are examined using laboratory mesocosms to determine the carbon-for-nutrient exchange efficiency of extant basal liverworts that form mycorrhiza-like associations with arbuscular mycorrhizal fungi, and to quantify rates of weathering of calcium and silicate minerals under liverworts and trees under ambient and early Palaeozoic atmospheric CO₂ concentrations.  Rates of photosynthate allocation to mycorrhizal fungi is shown to control their weathering rates.  In field experiments weathering of Ca from silicate and carbonate minerals increased with evolutionary advancement from liverworts to gymnosperm and angiosperm trees, and from arbuscular to ectomycorrhizal fungal partners. Our complementary geochemical analysis of Middle Devonian palaeosol cores preserving the root traces of one the Earth’s earliest forest ecosystems with cladoxylopsids and archaeophytalean progymnosperms, confirms the importance of rooting depth in pedogenesis and weathering.  Our findings support an over-arching hypothesis that evolution has selected plant and mycorrhizal partnerships that have intensified mineral weathering and altered global biogeochemical cycles.