Managing Global Resources for a Secure Future

High arsenic (As) levels in rice grains are a health concern for more than half of the world’s population who depend on rice for food, since inorganic As has been classified as a class 1 carcinogen by the International Agency for Research on Cancer. As accumulation in rice grains is greatly influenced by irrigation management and Alternate Wetting and Drying (AWD) has been reported to decrease grain As concentration compared to Continuously Flooded (CF) irrigation. The objective of this study was to determine how the timing and duration of AWD drying periods impacted grain As. Results from a 2-year field experiment comparing AWD to CF show grain As concentration in polished rice decreased on average by 48% in AWD when the soil was dried to 35% volumetric water content (water potential of -41 to -95 kPa) before being reflooded, which corresponded to a drying period of 10 – 12 days. Drying the soil longer did not translate into a further decrease in As content in rice grains. In contrast, drying the soil until the perched water table reached 15 cm below the soil surface (threshold used in “Safe AWD”) had no effect on grain As concentration; this may be attributed to the soil being close to saturation (water potential close to 0 kPa) before reflooding. Within drying periods of the same duration, grain As concentrations were lower when the drying period was imposed during panicle initiation and booting, compared to heading stage. Importantly, grain yields were not significantly different across all treatments. These findings need to be verified in other soil types before being extrapolated.