Arsenic in rice straw phytoliths: Encapsulation and release properties

Rice plants assimilate both silicon (Si) and arsenic (As) from the soil solution and accumulate these elements in their tissues. While Si has been known as a major material that builds up a so-called phytolith structure throughout the plant body, the fate of As in rice plants is not fully understood. This study aims to clarify the possible encapsulation of As into the phytolith structure in rice straw or rice straw ash and to evaluate the dissolution properties of the encapsulated As. Rice straw contains up to ~12 mg As kg−1, which highlights this material as a sink and source of As that needs to be taken into account. The intensified heat treatments of rice straw could lead to an enrichment of As in the burned products, from which As is likely associated more tightly with the phytolith structure. We observed strong correlations of released As and Si, demonstrating that the release of As is tightly controlled by the dissolution of phytoliths. This means that phytoliths can contribute to the soil As pool, but this As source might be obscured because it is recalcitrantly lodged within the phytolith structure. This finding serves as a basis for further assessing the role of phytolith-encapsulated As rice paddy systems and for developing proactive practices to manage As-tainted straw.