Solubility–crystallinity relationship of phytoliths derived from different species and organs of gramineous plants

Aims Silicon (Si) is a beneficial element for gramineous plants. Si in soil primarily originates from soil minerals but plant-derived phytoliths have recently attracted attention as an important alternative source of available Si. As the controlling factors of solubility of phytoliths remain unclear, this study investigated the solubility of phytoliths from different species and organs of gramineous plants, clarifying the physicochemical factors determining their solubility. Methods Phytoliths were prepared from the culms, branches, and leaves of moso bamboo ( Phyllostachys pubescens ), the stems and leaves of sugarcane ( Saccharum officinarum ), and the straw and husks of rice ( Oryza sativa ) using a wet digestion method. The solubility of the phytoliths was determined via 0.2-M NaOH extraction. Their crystallinity was evaluated based on X-ray diffraction analysis. Results The initial Si dissolution rate and maximum Si dissolution amount of the phytoliths ranged from 71.1 to 205 g Si kg⁻¹ h⁻¹ and 223 to 429 g Si kg⁻¹, respectively. Both parameters exhibited an increasing trend from the proximal to distal parts. The amounts of the opal-A and α-quartz of the phytoliths ranged from 66.7% to 97.5% and from 3.5% to 7.8%, with a negative correlation (p < 0.01). The opal-A increased from the proximal to distal parts of the plants. The initial dissolution rate and maximum dissolution amount exhibited a positive and negative correlation with the opal-A and α-quartz, respectively (p < 0.01). Conclusion These findings elucidate the mechanism controlling Si supply from phytoliths and provide fundamental insights into their role in Si cycling in soils.