Impact of Alternate Wetting and Drying Irrigation on Arsenic Uptake and Speciation in Flooded Rice Systems

Abstract

Alternate wetting and drying (AWD) irrigation can be used to promote oxic soil conditions and decrease arsenic (As) mobility and uptake into rice plants. However, scant information is available quantifying plant As speciation and uptake at the field scale for AWD with different soil drying severities. It is hypothesized that as the severity of soil drying increases, plant uptake and subsequent accumulation of both inorganic and organic As in the grain will decrease. However, since AWD can increase cadmium (Cd) bioavailability, Cd concentrations in rice grains should be evaluated concomitant to As. In this two-year field study, As and Cd uptake were examined, with routine plant and water sampling during the growing seasons, under three AWD practices varying in soil drying severity (from most to least severe: AWD25: drying to 25% volumetric water content at the root zone; AWD35: to 35%; AWDS: Safe AWD, drying to perched water table 15 cm below the soil surface), compared to a continuous flooding (CF) control. Arsenic speciation was also analyzed in grain and vegetative tissues. AWD25 and AWD35 decreased As accumulation in roots and straws by a similar amount compared to CF, leading to a 41-68% decrease in grain total As concentration. Speciation analysis revealed that AWD25 and AWD35 decreased grain concentration of organic As by 70-100% and inorganic As by 14-61% compared to CF. In contrast, AWDS did not decrease As uptake by rice compared to CF. Grain Cd levels were 6.5 μg kg−1 in CF, 16.6 μg kg−1 in AWD35, and 27.4 μg kg−1 in AWD25, suggesting AWD35 could serve as a mitigation option for As, while minimizing Cd accumulation.