Phytolith biogeochemistry and silicon regulation of terrestrial biogeochemical carbon cycle

Phytoliths in most terrestrial plant tissues as a result of silica biomineralization may occlude 0.1–6% of organic carbon (C). Phytolith-occluded carbon (PhytOC) comes mainly from photosynthesis and can be stable in soil and sediment environments for several hundred to thousand years. Phytolith turnover may influence terrestrial biogeochemical C cycle either directly through phytolith C sequestration or indirectly through regulating plant biomass C composition and accumulation, and soil organic carbon (SOC) stability. Phytolith C sequestration rates in terrestrial ecosystems of China increase in the following order: grasslands < forests < croplands. Active management practices including cultivation of silicon (Si)-rich plants and amendment of Si-rich materials (e.g., basalt powder and biochar) to increase aboveground net primary productivity (ANPP) and Si supply can significantly increase phytolith C sequestration. The dissolved Si from silicate weathering and phytolith dissolution can decrease plant lignin content and increase the accumulation of plant biomass C through mitigating abiotic and biotic stresses and improving stoichiometry of C, nitrogen (N) and phosphorus (P). The recovery of plant biomass C in response to Si accumulation usually exhibits an S-shaped curve under biotic stress and a bell-shaped curve under abiotic stresses. Generally, Si can recover approximately 30 to 40% of plant biomass C under abiotic and biotic stresses. Phytolith and related dissolved Si in soils can increase SOC stability through phytolith adsorption, Si and aluminum interaction, and Si and iron interaction.