How important is carbon sequestration in phytoliths within the soil?

Background and aims An overlooked fraction of the terrestrial carbon (C) pool is that associated with biogenic silica deposited in plants (phytoliths), so-called PhytOC. This fraction is small compared with the main C pools, but is of interest because it could be a long-term C sink as phytoliths may protect organic C from mineralization. However, the topic is hotly contested and unclear due to both methodological and theoretical limitations. Scope We aim to review this topic, with specific emphasis on: (i) the range of C concentrations associated with phytoliths; (ii) soil phytolith preservation and subsequent organic C mineralization; and (iii) global estimates of C sequestration within PhytOC. Conclusions Recent work has suggested that [PhytOC] could be much greater than currently acknowledged, but also highly variable and dependent on cell silicification types. A short case study using cryo-Scanning Electron Microscopy (cryo-SEM), X-ray microanalysis (EDX), plus Focused Ion Beam (FIB) and Scanning Transmission Electron Microscopy (STEM) on the culms of a sedge (Schoenus caespititius) confirmed this thinking. Understanding of both phytolith and PhytOC fates in soil is poor. We suggest that phytolith residence time should be seen as a gradient. Such a continuum is explained by different phytolith sizes, types and chemistry, which will also have contrasting PhytOC. Our estimation of C sequestration as PhytOC each year (11-190 Tg C yr(-1)) represents between < 1% and 13% of the C that could be sequestered globally in soils (estimated at 1400 Tg C yr(-1)). We conclude that (1) more research is needed to improve our understanding of the formation and fate of PhytOC in terrestrial ecosystems and (2) it would be unwise to put our faith in PhytOC sequestration or other related methodologies to "solve" the climate crisis.