Possible source of ancient carbon in phytolith concentrates from harvested grasses

K. K. Treseder,A. Alexandre,R. Corbineau,P. E. Reyerson,J. R. Southon,G. M. Santos

doi:10.5194/bg-9-1873-2012

Abstract

Abstract. Plants absorb and transport silicon (Si) from soil, and precipitation of Si within the living plants results in micrometric amorphous biosilica particles known as phytoliths. During phytolith formation, a small amount of carbon (<2%) can become occluded in the silica structure (phytC) and therefore protected from degradation by the environment after plant tissue decomposition. Since the major C source within plants is from atmospheric carbon dioxide (CO2) via photosynthesis, the current understanding is that the radiocarbon (14C) content of phytC should reflect the 14C content of atmospheric CO2 at the time the plant is growing. This assumption was recently challenged by 14C data from phytoliths extracted from living grasses that yielded ages of several thousand years (2–8 kyr BP; in radiocarbon years "Before Present" (BP), "Present" being defined as 1950). Because plants can take up small amounts of C of varying ages from soils (e.g., during nutrient acquisition), we hypothesized that this transported C within the plant tissue could be attached to or even embedded in phytoliths. In this work, we explore this hypothesis by reviewing previously published data on biosilica mineralization and plant nutrient acquisition as well as by evaluating the efficiency of phytolith extraction protocols from scanning electron microscope (SEM) images and energy dispersive spectrometer (EDS) analyses from harvested grasses phytolith concentrates. We show that current extraction protocols are inefficient since they do not entirely remove recalcitrant forms of C from plant tissue. Consequently, material previously measured as "phytC" may contain at least some fraction of soil-derived C (likely radiocarbon-old) taken up by roots. We also suggest a novel interpretation for at least some of the phytC – which enters via the root pathway during nutrient acquisition – that may help to explain the old ages previously obtained from phytolith concentrates.

Highlights

Phytoliths are amorphous silica particles that precipitate in and/or between the cells of living plants
Details of the phytolith extractions, blank assessment and the examination of C sources contaminants have been addressed elsewhere (Santos et al, 2010a). These intriguing 14C results raised the following questions: (a) can a chemical extraction procedure that does not use solvents or plastic devices affect the final 14C results?; (b) can root uptake be a source of old C that may be recalcitrant to the extraction procedures, and remain on the phytolith concentrates?; and (c) what are the sources of phytC itself: carbon fixed solely via photosynthesis, or taken up by roots, or both? In this overview paper, we will attempt to answer these questions by reviewing the literature on the silicification process and its functions, as well as nutrient acquisition and inorganic C incorporation by roots versus photosynthesis
This old component cannot account for more than a few percent of the whole plant C, 14C ages of several thousand years previously obtained from harvested grass phytoliths concentrates (Santos et al, 2010a) suggest that it may become occluded in or attached to the biogenic amorphous silica structure

Summary

Introduction

Phytoliths are amorphous silica particles that precipitate in and/or between the cells of living plants. Contrary to previous efforts to directly date phytC, this work included a very comprehensive blank assessment for the phytolith extraction procedure, involving a suite of 14C measurements on modern and 14C-free materials that can mimic phytolith structure, which showed that exogenous C associated with the chemical extraction was ∼3 μg of modern and ∼2 μg of dead (14C-free) carbon (n = 10). Details of the phytolith extractions, blank assessment and the examination of C sources contaminants have been addressed elsewhere (Santos et al, 2010a) These intriguing 14C results raised the following questions: (a) can a chemical extraction procedure that does not use solvents or plastic devices (possible sources of older exogenous C) affect the final 14C results?; (b) can root uptake be a source of old C that may be recalcitrant to the extraction procedures, and remain on the phytolith concentrates?; and (c) what are the sources of phytC itself: carbon fixed solely via photosynthesis, or taken up by roots, or both? We will discuss how current phytolith chemical extraction procedures for isotopic analyses of C may be inadequate, because they appear to leave organic compound residues in phytolith concentrates

The silicification process and its roles

Isotope analyses of phytC

Organic residues in phytoliths concentrates from harvested grasses

Discussion

Findings

Conclusions

Future directions