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Title: Effects of evolutionary advances in plant-mycorrhiza associations on biological weathering
Author: Quirk, Joe
ISNI:       0000 0004 2719 9427
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2012
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The evolution of Earth's first forests during the Devonian and expansion of angiosperrns at the expense of gymnosperms in the Cretaceous is thought to have accelerated the dissolution of continental Ca-Mg silicates. thereby enhancing long-term atmospheric CO2 draw-down and global climatic cooling. However, this paradigm of plant-driven atmospheric CO2 decline overlooks the role of eo-evolving arbuscular mycorrhizal (AM) soil fungi that have formed symbiotic partnerships with plants since the dawn of terrestrial plant life. and ectomycorrhizal (ECM) fungi whose evolutionary origins date from the Cretaceous. Both AM and ECM increase the mobilisation and uptake of elements from rocks and minerals. which they provide to their host plants in exchange for carbon energy. Although the involvement of later-evolving ECM in weathering is increasingly recognised, the role of AM has been overlooked or even prematurely dismissed. Here. evidence is presented of the effects of the co-evolutionary advances made by plants and mycorrhizal fungi on the weathering of silicates that in turn has been instrumental in modifying Earth's atmospheric CO2 over geological time. Whilst accounting for the functional type, leaf habit and mycorrhizal status of the hosts. this study addresses the missing mechanistic understanding of the impact that the major evolutionary advances from gymnosperm to anglosperm and AM to ECM have had on weathering processes over the last 400 Myr. Within this study. the relative importance of AM and ECM fungal-mineral interactions are examined using established trees in the field. as well as in fully controlled laboratory studies at ambient, sub- and super-ambient atmospheric CO2• The efficacy of mycorrhizal weathering is also assessed in comparison with that of plants which have evolved beyond the need for mycorrhizas in favour of alternative nutrient- acquisition strategies involving specialised roots. Finally. the effect of low atmospheric CO2 concentrations on drought-induced plant mortality is investigated as a mechanism attenuating silicate weathering by woody plants. potentially limiting atmospheric CO2 drawdown over the past 24 Myr. Three crucial gaps in our understanding of plant-mycorrhiza weathering are addressed: the ability of AM versus ECM fungi to colonise Ca-Mg silicate rocks such as basalt. their ability to physically alter minerals. and their capacity to chemically modify silicate rock. The results demonstrate that weathering beneath trees is fundamentally determined by co-evolutionary partnerships with mycorrhizal fungi. and provide critical lnsights of the geochemical consequences of the rise of the first forests in the Devonian and expansion of ECM gymnosperms and angiosperms at the expense of AM gymnosperms from the Cretaceous.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available