83-2 A novel method for measuring phytolith carbon concentrations reveals the impact of phytoliths on carbon sequestration

Abstract:

Phytoliths, microscopic silica bodies (~10–40 μm), form when living plants take up monosilicic acid through their roots and deposit them in their tissues. They are particularly abundant in Poaceae (grasses) and form a sizable silica pool in grassland soils after grass tissue decays. Phytoliths incorporate carbon in their silica matrix, so-called phytolith-occluded carbon (PhytOC). Whether PhytOC contributes to the significant underground carbon storage in grasslands is debated, but has major implications for global carbon sequestration estimates, as >40% of terrestrial surfaces are currently covered by natural and agricultural grasslands. Bulk grass phytolith carbon content estimates range from 0.1% to >10%, resulting in major differences in modeled sequestration estimates. 

We present a novel approach for measuring individual phytolith carbon concentrations that uses secondary ionization mass spectrometry. We sampled four greenhouse-grown grasses and seven wild grasses collected in the Great Plains, USA, together covering a range of ecological settings (open- vs. closed-habitat grasses, C₃ vs. C₄). Grass silica short cell phytoliths (GSSCPs) were mounted in indium and analyzed using the CAMECA 1280 Ion Microprobe at University of Hawaiʻi at Mānoa. A spot size of 3–4 μm allowed for analyses within phytoliths without overlapping grain margins. A calibration curve was generated using a set of homogenized nanopowder mixtures of SiO₂ and C with concentrations from 0.11 to 2.8 wt% C. 

Measured carbon concentration in 175 phytoliths ranged from below detection (~0.01 wt% C) to above the calibration curve threshold. Average greenhouse-grown sample concentrations ranged from 0.04–1.3 wt% C, excluding Chusquea culeou (bambusoid), which exceeds measurement thresholds using our analytical setup. Species average concentrations in wild grasses ranged from 0.014–0.69 wt% C. Wild C₃ grasses have low PhytOC (0.014–0.21 wt% C) and low intra-phytolith variability. C₄ species contain higher wt% C (0.24–0.69 wt% C) and display more intra-phytolith variability. Broader taxonomic sampling and analysis of non-GSSCP phytoliths are needed to fully understand carbon concentration patterns within Poaceae. Our preliminary results indicate both a wide range of PhytOC values among grasses in general and the tendency for C₄ species to sequester more carbon than C₃ taxa. If this latter finding holds true with additional sampling, it will have tremendous implications for carbon cycle functioning and thus for climate mitigation strategies.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-22229

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