Increasing plant-availability of Si through soil amendment of Si-rich rice residues can decrease inorganic As without affecting Cd levels in grain under flooded soil conditions. However, the impacts of Si amendments on Cd and As uptake by rice under different flooding extents have not been reported. We investigated the effects of different flooding extent on As and Cd uptake by rice and accumulation in grain in well-weathered soil amended with Si-rich rice husk (Husk) or mixed charred/ashed rice husk (Ash). Our results show that Husk and to a lesser extent Ash amendments decreased grain As under both flooded (~40% and 20% decrease, respectively) and nonflooded (~75% decrease) conditions due to increased Si. Under flooded conditions grain As and yield is higher, and Husk amendment additionally decreased grain inorganic As by ~45%. Under nonflooded conditions grain Cd is higher and yield is lower, and Ash amendment decreased grain, husk, and straw Cd by ~40-50% not due to Si, but due to increased aboveground biomass and an increase in soil pH, which helped to retain Cd in soil. These data illustrate that rice residue addition to paddy soil can lower human health risk under both flooded and nonflooded conditions without affecting grain Zn and Fe.
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