Irrigation management for arsenic mitigation in rice grain: Timing and severity of a single soil drying

The accumulation of arsenic (As) in rice grain is a public health concern since As is toxic to humans; in particular, inorganic As can cause many chronic diseases including cancer. Rice crops are prone to accumulating As, in part, due to the anaerobic soil conditions triggered by the traditional continuously flooded irrigation practice. The objective of this study was to determine how the severity and the timing (i.e. crop stage) of a single soil drying period impact total As concentration and As speciation within the rice (both white and brown) grain, compared to a continuously flooded (CF) control. Drying the soil until the perched water table reached 15 cm below the soil surface (same severity as in the "Safe Alternate Wetting and Drying"), which in this study corresponded to a soil (0-15 cm) water potential of ~0, did not decrease grain As concentrations, regardless of timing. Drying the soil to Medium Severity [MS: soil (0-15 cm) water potential of -71 kPa] or High Severity [HS: soil (0-15 cm) water potential of -154 kPa] decreased total As by 41-61%. However, inorganic As did not always decrease because the severity and the timing of soil drying affected As speciation within the grain. Overall, the soil had to be dried to HS and/or late in the growing season (i.e., at booting or heading instead of at panicle initiation) to decrease inorganic As concentration in the rice grain. This study indicates that the imposition of a single soil drying period within the growing season can mitigate As accumulation in rice grain, but it depends on the severity and timing of the drying period. Further, irrigation management affects As speciation within the rice grain and this must be considered if regulations on inorganic As are based on a percentage of total As measured.