Controlling exposure to As and Cd from rice via irrigation management.

Irrigation management controls biogeochemical cycles in rice production. Under flooded paddy conditions, arsenic becomes plant-available as iron-reducing conditions ensue, while oxic conditions lead to increased plant availability of Cd in acidic soils. Because Cd enters rice through Mn transporters, we hypothesized that irrigation resulting in intermediate redox could simultaneously limit both As and Cd in rice grain due to As retention in soil and Mn competition for Cd uptake. In a 2 year field study, we used 6 irrigation managements that varied in extent and frequency of inundation, and we observed strong effects of irrigation management on porewater chemistry, soil redox potentials, plant As and Cd concentrations, plant nutrient concentrations, and methane emissions. Plant As decreased with drier irrigation management, but in the grain this effect was stronger for organic As than for inorganic As. Grain organic As, but not inorganic As, was strongly and positively correlated with cumulative methane emissions. Conversely, plant Cd increased under more aerobic irrigation management and grain Cd was negatively correlated with porewater Mn. A hazard index approach showed that in the tested soil with low levels of As and Cd (5.4 and 0.072 mg/kg, respectively), irrigation management could not simultaneously decrease grain As and Cd. Many soil properties, such as reducible As, available Cd, soil pH, available S, and soil organic matter should be considered when attempting to optimize irrigation management when the goal is decreasing the risk of As and Cd in rice grain.