Arsenic Dynamics in Paddy Rice Ecosystems and Human Exposure

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Rice is the staple diet for more than 3.5 billion people around the world. Elevation of arsenic (As) in paddy rice ecosystems has become an environmental, economic, and public concern due to its adverse consequences on global rice production, food safety, and human health. Mining exploitation, weathering of As-bearing minerals, dissolution of aquifer sediments, and As-contaminated groundwater irrigation have extensively contributed to the contamination of paddy soil by As in high levels. Speciation, mobility and sequestration of As in paddy soil–water interfaces are controlled by iron (Fe) plaque formation, redox sensitive mineral surfaces (Fe and Mn), organic matter and competing substances (PO $$_{4}^{3-}$$ and Si(OH)4). During flooding season, paddy soil porewater is contaminated with high concentrations of inorganic As species, particularly by more toxic arsenite (As(III)) as result of arsenate (As(V)) reduction under anaerobic conditions. Microorganisms play a crucial role in As speciation dynamics promoting redox transformation, methylation and volatilization processes. Various metabolic pathways, including As(V) reduction, As(III) efflux, and As(III)-thiol complexation govern As uptake, translocation, and loading into rice grains. The translocation of As from rice root to shoot leads to the accumulation of toxic As species in grains affecting rice quality and yield. The worst scenario of grain As is associated with the human exposure to high amounts of As via consumption of As-contaminated rice and related food products. Hence, this chapter provides an overview of (i) As speciation and transformation dynamics, (ii) As uptake mechanisms from root to shoot, (iii) As metabolic pathways over root, shoot and grain loading, and (iv) recommendations for future research. Such a widespread understanding of As dynamics in paddy rice ecosystems is crucial to develop sustainable As mitigation strategies and alleviate adverse impacts on global food safety and human health.