Articles | Volume 13, issue 4
Research article
01 Mar 2016
Research article |  | 01 Mar 2016

Unambiguous evidence of old soil carbon in grass biosilica particles

Paul E. Reyerson, Anne Alexandre, Araks Harutyunyan, Remi Corbineau, Hector A. Martinez De La Torre, Franz Badeck, Luigi Cattivelli, and Guaciara M. Santos

Abstract. 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 chemical-thermal 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.

Short summary
We characterize the origin of carbon (C) in phytoliths (biosilica of higher plants) by a multi-isotope approach. We show that phytoliths occlude soil organic C adsorbed through the roots, making them unsuitable for paleo-proxy studies, 14C dating or atmospheric CO2 sequestration. Our findings are in parallel with recent soil paradigm shifts showing that soil microbes access old C and therefore call for further investigations on the role of old C in root–plant interactions and biomineralization.
Final-revised paper