Dynamic Silicon-Carbon Interactions in Terrestrial Vegetation

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The global carbon balance has been the focus of much research recently because of its primary influence on global climate. Considerable work has gone into understanding and identifying feedbacks within the global carbon cycle (C-cycle), but what has not yet been investigated sufficiently is the interaction with other biogeochemical cycles. For example, the cycling of silicon (Si) is important for the C-cycle because the weathering of silicate rocks removes CO2 from the atmosphere. Furthermore, productivity of marine plants (particularly diatoms) is limited by Si and therefore anything that influences the strength of the "biological CO2 pump" (i.e. removal of atmospheric CO2 by ocean primary producers) influences concentration of CO2 in the atmosphere. In addition to plants influencing the rate of Si weathering, many plants are accumulators of weathered Si; these plants use Si-based compounds to enhance stem rigidity or to improve defences against disease and herbivores. Most biogeochemical models of Si cycling omit the role of terrestrial plants or treat all terrestrial ecosystems as if they are the same. It is the overall objective of this research grant, therefore, to introduce terrestrial plant-Si processes into global C-Si cycle models. We will never be able to get the global carbon cycle to balance if we don't consider feedbacks from the Si cycle, and we will never get the Si cycle right if we omit the entire terrestrial biosphere. There does exist growing literature on the ecophysiological basis of plant-Si interactions that could be quantitatively analyzed and incorporated into existing biogeochemical models. Data addressing the mechanisms involved in plant uptake of Si and the processes involved in the decomposition of biogenic Si and leaching of Si will be statistically analyzed with the intent to mechanistically model the most significant of these biological processes. Being able to place the cycling of Si in an evolutionary context will also be useful for understanding its role in altering the global C-cycle through geological time. The end-Cretaceous marks a turning point for terrestrial vegetation because the evolution of flowering plants (angiosperms) is thought to have led to the replacement of existing gymnosperm (conifer)-dominated ecosystems. In what ways did the evolution of angiosperms and some of the Earth's largest Si plant accumulators influence global C-cycling? Did the ability of angiosperms to accumulate Si (and use it as a cheap alternative to carbon) change the competitive balance between angiosperms and gymnosperms? Such questions will form the basis of research proposed for NSERC funding. The impact of the proposed research is substantial because we currently do not consider the terrestrial biosphere as an active component of Si-cycling therefore will never be able to fully understand the global carbon cycle.