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

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 gains
b. How ecological stoichiometry and nutrient acquisition pathways vary across the land plant phylogeny
c. Relationships between species, symbiont and mineral weathering rates

Second, 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.

Our research proposal addresses fundamental knowledge gaps regarding the influence of the evolution of the terrestrial biosphere on Earth's global climate. Using a combination of field work, laboratory experiments and a new type of computer model, our project will test the hypothesis that the evolving terrestrial biosphere and, specifically, its effects on phosphorus and nitrogen cycling was a major contributor to climate change over the Phanerozoic.

Our adventurous research aims align directly with NERC's strategic goals in advancing knowledge and predictive capabilities of how our planet works, appealing to and benefitting not only researchers and the wider scientific community, but also policy makers, lobby groups and the wider public. We will engage with these broad audiences to maximize the impact of our research through the following activities:

Climate policy workshop
We will deliver the first spatially-resolved long-term geochemical model, informed by key plant, symbiont and soil processes that regulate Earth's global climate with important potential impacts in developing effective climate policies at local and national governmental scales. Earth's response to future CO2-driven warming can be assessed by partnership between researchers in future climate change and those in paleoclimate modelling, and in turn, paleoclimate models can be validated by predicting changes in global geochemistry. We will hold a workshop in the final year of the project to facilitate such interactions and to foster new ones with key climate change and Earth system/geochemistry research groups (e.g. UK National Centre for Atmospheric Science, USA Princeton/NOAA-GFDL modelling community) and key stakeholders in climate change-related policy (e.g. the All-Party Parliamentary Climate Change Group, The Climate Coalition and Committee on Climate Change). We will provide critical information to these groups and define/facilitate the assimilation of our research outputs into local and national level public policy.

Eden Project engagement workshops
Building on our existing links with the Eden Project's science team, we we will develop a series of talks, demonstrations and interactive activities to communicate our research to the public each year of the project. These will involve a combination of real-time interactive experiments examining the symbiotic microbes and fungi at Eden, public talks, demonstrations showing how mycorrhizas and N-fixers work and guided walks around the Rock Garden ("the biology of geology") to discuss the role of "invisible" communities in the 'greening of the Earth'. This will be augmented by our project website, including visually-stimulating 'Evolving Earths'- moving maps form our computer model showing plant and climate evolution over time. Our target audience are members of the general public of all ages who are minded to visit the global gardens of the Eden Project who are likely to have an appreciation of the wider environment: we would seek to enhance this through our event.

Dedicated project website
Our fundamental science results and visually-arresting Evolving Earth reconstructions generated from them will foster educational engagement in Earth system science, evolution, plant-symbiont relationships and biosphere-climate interactions. Our project website will allow users to understand the complex interplays between plants, symbionts and global climate and will form an excellent tool to illustrate the importance of our research to broad audiences. It will also provide educational resources to teachers and school-age students in natural sciences. We will ensure curriculum-relevant content is signposted and highlighted to A-level/GCSE teachers and undergraduate lecturers by working with the Faculty of Biological Sciences and Faculty of Environment outreach teams who already engage with local schools/colleges, in addition to highlighting it through outreach events described above.