Silicates in the mycorrhizosphere : modelling the effect of mycorrhizal fungi on global weathering and the long-term carbon cycle

For the past two decades, the spread of angiosperm trees in the Cretaceous and Palaeogene has been thought to have enhanced the weathering of silicate minerals resulting in increased fluxes of Ca and Mg to the oceans, drawing down atmospheric CO2 and ultimately sequestering it in marine carbonate sediments. In this thesis, I present an alternative hypothesis: the spread of ectomycorrhizal fungi since the Cre- taceous has been more important for global weathering and drawdown of CO2 from the atmosphere than the spread of angiosperms. These fungi act as biosensors, seek- ing out nutrient-bearing minerals and releasing weathering agents such as protons and organic acids at the scale of individual mineral grains. By contrast, fine roots have diameters several orders of magnitude larger than the fungi and are therefore less able to target mineral grains smaller than themselves. I developed deterministic process-based models of silicate weathering mediated by roots and mycorrhizal fungi with the aim of examining their influence on delivery of Ca and Mg to river waters. My zero-dimensional global model results show that EM fungi could have accounted for the CO2 drawdown previously attributed to angiosperm trees, but results from my two-dimensional spatially-resolved model indicate a more nuanced picture, with AM and EM angiosperms playing an important role due to their high net primary productivity. This model also predicts that falling CO2 causes weathering rates to decline, supporting a lower limit for CO2 drawdown. Overall, the models indicate that biological weathering is a complex interplay between climate, lithology, and plant and mycorrhizal functional typesEThOS - Electronic Theses Online ServiceGBUnited Kingdo