Phytolith-Occluded Carbon Pools and Fluxes: New Estimates

Abstract:

Phytoliths are microscopic grains of silica (SiO₂∙nH₂O) formed within plants. The biomineralization process typically encapsulates small quantities of carbon termed phytC. Upon decomposition, phytoliths are released from biomass and into soils. Recent research has suggested that phytC may be a large sink of atmospheric CO₂ in soils. Important steps, therefore, are to quantify phytC cycling across ecosystems and to measure it’s importance relative to the organic C cycle as a whole. Thus, information regarding phytC pool sizes and flux rates are needed. To an extent this has been performed. PhytC quantities can be easily estimated as long as 1) phytolith quantities and 2) the amount of C present in phytoliths are known. The quantity of C within phytoliths is still a subject of debate, but recent work has found quantities of less than 0.22%. Older studies, which rely on extraction methods which are now known to incompletely remove surface organic residues, have found phytC quantities from 1% to 20%. Hence, studies of phytC cycling using outdated methods may lead to overestimates. In order to re-estimate phytC dynamics, we compiled an extensive list of published works which document phytolith pools in above-ground biomass and soils, as well as flux rates. From these data phytC quantities were calculated using revised estimates of phytolith C percentages. PhytC quantities were also compared to total organic C (TOC) pools and fluxes. These calculations were then extrapolated to biome and global scales. At the biome scale, our results indicate that phytC within living biomass and soil pools as well as fluxes are one to two orders of magnitude smaller than previously estimated. PhytC is generally less than 0.01% of biomass TOC, and less than 1% of soil TOC. Annual phytC fluxes are less than 0.01% of TOC fluxes. At the global scale, annual phytC production is approximately 0.01% to 0.10% of gross C production. The findings of the present study suggest that direct C sequestration by phytoliths may not be as significant as once thought. Recent research has found that at least some phytC originates from soil organic C, which raises further questions about the amount of C actually sequestered. This, coupled with the trace amounts of phytC existing in nature, demonstrate that this component of TOC is not a viable C sequestration mechanism.