Revealing the source of carbon occluded in phytoliths (phytC): potential implication concerning the role of plant Si cycling on carbon cycling

Project description: 
Silicon is taken up by the roots in its dissolved form, translocated in the sap, and deposited in the cells as micrometric hydrous amorphous silica particles, also called phytoliths. Phytoliths can contain small amounts of C (phytC) occluded in their structure and ranging from 0.1 to 2% of phytolith dry weight. It is generally assumed that the source of this phytC is atmospheric CO2 that was fixed by the plant via photosynthesis. However, the assumption that the source of phytC is 100% photosynthetic has been recently challenged by unexpected old AMS-14C ages obtained for phytoliths extracted from contemporary grasses (though bulk material from the same plants gave contemporary 14C values) (Santos et al., 2010). Because plants can take up small amounts of organic N from soils, especially through the action of mycorrhizal fungi, we hypothesized that soil organic compounds could be released during soil Si-bearing minerals dissolution in the rhizosphere, absorbed (under the form of chelates) by the plant during Si uptake, transported in the sap, and eventually concentrated and occluded in phytoliths during silica precipitation (Santos et al., 2012). In order to test this hypothesis we plan to investigate with the nanoSIMS if 13C and 15N can be detected in phytoliths from grasses grown in solutions enriched in 13C and 15N labeled molecules. Simultaneously, in order to investigate the potential role of native endomyccorhizae in old soil C potential uptake by the grass roots, we propose to analyze by AMS the 14C signature of phytC from grasses grown on old and young volcanic soils α-irradiated or not.