The regulatory role of CO 2 on nutrient releases from ashed rice straw phytoliths

Phytolith is widely known as a silica structure in numerous silicon (Si) accumulator plants, e.g., rice, and it contains various nutrients and other beneficial elements. 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 CO 2 , 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 800 °C were investigated by quantifying Si release together with other nutrients under two atmospheric conditions, i.e., without and with CO 2 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. CO 2 significantly reduced the dissolution rate of the phytolith ashes but increased the release rates of cationic as well as anionic nutrients. The aeration of CO 2 shifted the carbonate equilibrium (H 2 CO 3 , HCO 3 − and CO 3 2− ) towards H 2 CO 3 , reducing solution pH, thereby decreasing the dissolution rate of phytoliths. Following this, intensification of H + exchange promoted nutrient releases from the phytolith ashes. This indicates contrasted responses of phytoliths and their occluded nutrients to CO 2 , and provides a better insight on the fate of soil phytoliths and the tendency of nutrient budgets from rice straw phytoliths in soils.