Effect of Alternate Wetting and Drying Irrigation on Arsenic Speciation and Uptake in a Rice Paddy System.

Effect of alternative wetting and drying irrigation on arsenic speciation and uptake in a rice paddy system

Chongyang Li, Daniela Carrijo, Bruce Linquist, Peter G. Green, and Sanjai J. Parikh

The bioaccumulation of arsenic (As) into rice has received growing attention in recent years and proactive steps to minimize uptake are now needed. One irrigation management approach, alternate wetting and drying (AWD), is to subject rice fields to intermittent flooding and draining periods. AWD has been shown to have benefits as water-saving and greenhouse gas (GHG) emission reduction strategies, and may reduce As uptake by preventing accumulations of reduced As (As^III) in soil and water. To investigate the arsenic uptake into rice under different irrigation strategies, three irrigation practices, including 25% moisture content by volume (MCV), 35% MCV and continuous flooding (CF), were conducted for one growing season in Northern California. Water, soil, plant shoots and roots were sampled throughout the growing season and rice grains were collected at harvest. As analysis (total, speciation) of the plant tissues and environmental matrices was performed by nitric acid digestion and liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). Results indicated that total arsenic level in bulk soil did not significantly change throughout the growing season (3.90 ± 0.07 mg/kg to 3.79 ± 0.03 mg/kg) across treatments. Likewise, total As in water samples did not drastically fluctuate with time (0.34 ng/g to 1.57 ng/g), although slightly higher As^III levels in water under the CF treatment was observed before harvest. Uptake of As by rice roots, however, was effectively hindered under two AWD treatments during the entire season. Lower As in roots correlated with less arsenic accumulation in above ground biomass (shoots and grains). Total As in both rice and brown rice was reduced by 36% for the 25% MCV compared to CF. Grains harvested under AWD treatments contained only As^III while those under CF treatment had both As^III and less toxic dimethyl arsenic (DMA) with significantly higher level of As^III than AWD treatments.