How did the evolution of plants, microbial symbionts and terrestrial nutrient cycles change Earth's long-term climate?

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The Phanerozoic Eon (the last 540 million years) encompasses the evolutionary history of land plants from the initial colonization of the land through to forests and flowering plants. Earth's climate has undergone major changes over this timeframe, but it remains uncertain whether these changes were primarily driven by revolutions in the terrestrial biosphere, or by tectonic factors such as volcanic degassing of CO2. Resolution of this question lies at the heart of our understanding of how our planet operates, but the ability to answer it has been hampered by a lack of representation of the terrestrial biosphere in our biogeochemical computer models. These 'deep-time' models need to be simple in order to compute very long timescales, and this limits the ability to include spatial features such as locations of rainfall, which are vital to terrestrial modelling. A perhaps more fundamental problem is the lack of understanding of the way that plant evolution has altered global chemical cycling through changes to carbon-nitrogen-phosphorus ratios in tissue, and what the contribution of fungal and microbial symbionts were to supplying key limiting nutrients. This project brings together expertise in computer science, geochemistry, ecology and plant-symbiont physiology to build a new deep-time spatial Earth system model, informed by a targeted suite of plant growth experiments and a robust literature review.Firstly, we will run laboratory experiments with early diverging plants and symbiotic nitrogen-fixing trees, with and without partnership with fungal and/or nitrogen-fixing symbionts in microcosms with controlled atmospheric CO2 concentrations. Introduction of isotopically-labeled carbon, nitrogen and phosphorus will allow us to capture the carbon-nitrogen-phosphorus stoichiometric ratios and nutrient acquisition pathways for diverse plant-symbiont partnerships across the plant phylogeny, filling significant gaps in current knowledge of these processes. These experiments will allow us to understand:a. Plant-symbiont carbon-nutrient "costs" and "benefits" in terms of plant-fixed carbon and symbiont-acquired nutrient gainsb. How ecological stoichiometry and nutrient acquisition pathways vary across the land plant phylogenyc. Relationships between species, symbiont and mineral weathering ratesSecond, we will develop our new Earth system model. Here we will build on the framework of the 'COPSE' model (Carbon Oxygen Phosphorus Sulphur Evolution), which is arguably the most complete predictive 'deep time' box model in the literature, and which PI Mills has had a key role in developing over the last decade. A prototype fast spatial land surface module has been developed utilizing matrices in MATLAB and in this project we will couple the spatial land surface module to COPSE. This will allow us to build a dynamical representation of the evolving terrestrial biosphere, based both on our laboratory experiments and on literature vegetation models. This model will map the flows of phosphorus, nitrogen and carbon through the terrestrial system over geological timescales. Comparison of model outputs with multiple independent geochemical proxies will allow us to explore (1) how plant evolution and the development of symbiotic partnerships feeds back on Earth's climate; (2) the key evolutionary events that occurred through time and whether they can explain prominent CO2 drawdown events, such as during the Ordovician and Cenozoic; and, (3) the relative roles of the terrestrial biosphere vs. tectonics in controlling Earth's climatic history.Beyond the immediate results, the hybrid model we create will bridge the gap between box modelling of global geochemistry and true paleoclimate general circulation modelling, providing a useful tool for the community to further extend and employ.