The Regulatory Role of CO2 on Nutrient Releases from Rice-Straw Phytoliths

Abstract Phytolith has been widely known as a porous silica structure in numerous silicon (Si) accumulator plants, e.g., rice, that contain various nutrients and other beneficial elements too. When rice straw is returned to paddy fields, the silica structure of phytolith can be dissolved, thereby releasing its occluded nutrients. While the intrinsic characteristics and dissolution properties of phytoliths under the effect of solution chemistry have been intensively studied, the effect of gas phases, especially CO2, on phytolith stability and the implications for nutrient release are not fully known. Here, dissolution properties of phytolith ashes obtained from dry ashing of rice straw at 400, 600 and 800οC were investigated by quantifying Si release together with other nutrients under two atmospheric conditions, i.e., without and with CO2 support (aeration). In a time span of 6 days, the releases of nutrients (K, P, Ca, Mg) showed high dependence on the overall dissolution of the phytolith ashes. CO2 significantly reduced the dissolution rate of the phytolith ashes but increased the release rates of cationic as well as anionic nutrients. The aeration of CO2 shifted the carbonate equilibrium (H2CO3, HCO3− and CO32−) towards H2CO3, reducing solution pH, thereby decreasing the dissolution rate of phytoliths. Following this, intensification of H+ exchange promoted vast evacuation of cationic nutrients from the phytolith ashes. This indicates contrast responses of phytoliths and their occluded nutrients to CO2, and provides a better insight on the fate of soil phytoliths and the tendency of nutrient budgets from rice straw phytoliths in soils.