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Plant biosilica particles (phytoliths) contain small amounts of carbon called phytC. Based on the assumptions that phytC is of photosynthetic origin and a closed system, claims were recently made that phytoliths from several agriculturally important monocotyledonous species play a significant role in atmospheric CO2 sequestration. However, anomalous phytC radiocarbon (14C) dates suggested contributions from a non-photosynthetic source to phytC. Here we address this non-photosynthetic source hypothesis using comparative isotopic measurements (14C and δ13C) of phytC, plant tissues, atmospheric CO2, and soil organic matter. State-of-the-art methods assured phytolith purity, while sequential stepwise-combustion revealed complex chemicalthermal decomposability properties of phytC. Although photosynthesis is the main source of carbon in plant tissue, it was found that phytC is partially derived from soil carbon that can be several thousand years old. The fact that phytC is not uniquely constituted of photosynthetic C limits the usefulness of phytC either as a dating tool or as a significant sink of atmospheric CO2. It additionally calls for further experiments to investigate how SOM-derived C is accessible to roots and accumulates in plant biosilica, for a better understanding of the mechanistic processes underlying the silicon biomineralization process in higher plants. [ABSTRACT FROM AUTHOR]

Abstract: Phytoliths are micrometric particles of amorphous silica that form inside or between the cells of higher plant tissues throughout the life of a plant. Phytolith morphological assemblages extracted from sediments and buried soils are increasingly used as proxies of grassland diversity and tree cover density. When found in significant amounts in archeological sites they can be used for identifying food habits, cultural and agricultural practices. Phytoliths can contain small amounts of C occluded in their structure (phytC). It is generally assumed that the source of this phytC is atmospheric CO2 that was fixed by the plant via photosynthesis. Isotopic analyses of phytoliths (δ13C, 14C) were thus expected to inform respectively on the photosynthetic pathway or on the age of the mineralized host plants. However recent 14C analyses of phytC from phytolith concentrates extracted from soils and harvested grasses yielded unexpected 14C ages of several hundreds to kyr old. These 14C phytC results raised the question of a possible source of refractory/old soil organic matter component taken up by roots, which can be attached or occluded in phytoliths. Simultaneously these results highlighted the need for setting standardized protocols leading to concentrates entirely devoid of organic residues, as well as for a robust method for checking phytolith purity. The goal of this work was thus to develop protocols for extracting phytoliths from plants, leading to 100% phytolith purity, as required for phytC analyses. Protocol 1 utilizes a multi-step process of dry ashing and acid digestion, while protocol 2 also uses acid digestion as well as a separate alkali immersion step which removes surface layers. Phytolith concentrate purity was gauged in a semi-quantitative fashion through the use of SEM–EDS analysis. This quality check for phytolith purity can reveal small C particulate contamination of phytolith concentrates that may considerably bias isotopic and quantitative analyses of phytC. Results indicate that the two protocols were able to entirely remove small C particulate contamination. Protocol 1 produced phytolith concentrates with well defined morphologies suitable for both morphological and isotopic analyses. However measurement of C yields showed that protocol 1 probably induced C leakage, leading to lower recovery. Protocol 2 is faster, leads to higher C yield but may lead to a beginning of dissolution. With these protocols on hand, sources of phytC can be properly investigated. [Copyright &y& Elsevier]

Abstract: Researchers frequently assume that phytolith assemblages in modern soils reflect composition of recent vegetation because of the direct deposition of silica into the soil, once plants decay. This paper tests this assumption and determines whether temperate grasslands of different composition can be reliably detected based on their silica record in topsoil in a controlled experiment. The differences in total biogenic opal concentration (TBOC) and diversity of morphotypes were assessed in the Biodiversity II experiment (E120) at Cedar Creek Ecosystem Science Reserve, Minnesota, USA, where controlled mixtures of C3 grasses, C4 grasses, legumes, non-legume forbs, and woody shrubs (Quercus) were grown for a period of eight years. The plots have been manually maintained to contain the target species, and thus provide an opportunity to test numerous hypotheses regarding phytolith production patterns under diverse mixtures of plants. Soil samples were obtained from plots representing a variety of functional group mixtures. Pinch soil samples of 20 g from 10 random locations inside each plot were obtained. Phytoliths were extracted from each sample by chemically removing organics and carbonates and using heavy liquid flotation. A chemical dissolution method was used to obtain estimates of TBOC. Morphotypes were counted under a microscope. Morphotypes were analyzed on all plots against each other and against the morphotypes expected in the plants that grow on each plot using ANOVAs, linear regression, PCA and cluster analysis. Average above-ground biomass of expected phytolith producers was weakly but positively correlated with the TBOC values (R 2 = 0.42). The morphotype analysis showed that species'' composition was most accurately reflected in the phytolith assemblages on grass-dominated plots. For example, it was possible to distinguish C3, C4, or mixed grass-dominated plots from each other. Although the majority of phytoliths were from Poaceae, large shares were also from forbs and woody plants. Plots without any grasses still had some presence of grass phytoliths suggesting limited horizontal translocation and/or inheritance. [Copyright &y& Elsevier]