Plants absorb the chemical element silicon from soil through their roots and transport it into their aboveground parts. The deposit of silicon results in minute structures inside the plant known as phytoliths. During phytolith formation, small amounts of carbon and other elements can also become trapped and in the process become physically protected after the plant dies and undergoes decomposition. Since photosynthesis is the main way that plants acquire carbon to make their own food, it has been assumed that phytolith carbon only comes from the air and that the properties of carbon trapped in phytoliths should reflect those of the atmosphere at the time the plant was growing and fixing carbon into its leaves. In fact, building up phytolith-protected carbon in soils was recently considered as a possible way for removing some of the excess carbon in the atmosphere. However, the assumption that phytolith carbon only comes from the air has been recently challenged by the dating of carbon in phytoliths extracted from live grasses that has been found to be several thousands of years old. Because plants also take up small amounts of carbon from soils during nutrient uptake, the unexpected old ages of some of the phytolith carbon suggest that soil carbon could become trapped in phytoliths as well. If this is the case, opportunities for sequestering carbon via phytoliths are probably overestimated. To test these ideas, this project will analyze phytoliths extracted from grasses exposed to distinctly labeled carbon isotopes in two experiments. The results should reveal the origin - root uptake from soil or leaf uptake from the atmosphere - of the carbon in phytoliths. Additionally, an independent verification method using nanoparticles will be developed and applied.
Atmospheric carbon dioxide concentrations have increased dramatically since the Industrial Revolution. The recent interest in sequestering atmospheric carbon by promoting plants capable of producing large amounts of phytoliths makes it essential to evaluate the origin of phytolith carbon. If phytoliths are merely recycling carbon from soil, then their potential for additional sequestration is minimal. To test this, a new phytolith extraction method for reliably obtaining isotope results will need to be developed. Since many fields of science rely on isotope measurements of carbon inside plants, they will be informed by these findings. In addition, doctoral students in the United States and France will work in tandem to evaluate the new procedures. A postdoctoral fellow will work on the nanotechnology part of the project. Results will be used in annual short courses open to all undergraduate students at the University of California in Irvine, and also available to both national and international established researchers. Finally, the methods and results will be presented at meetings, incorporated into a text book, and publically released on a project web page.
