Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2011
Publication Date: 9/2/2011
Citation: Somenahally, A.C., Hollister, E.B., Yan, W., Gentry, T.J., Loeppert, R. 2011. Water management impacts on arsenic speciation and iron-reducing bacteria in contrasting rice-rhizosphere compartments. Journal of Environmental Science and Technology. 45:8328-8335. Interpretive Summary: Arsenic (As) is a toxic metalloid known to cause cancer in humans. Rice is extensively cultivated on As-impacted soils throughout the world, resulting in high grain-As concentrations. As a result, the consumption of As-impacted rice, apart from drinking water, can be an additional exposure route for millions of people worldwide. The rice rhizosphere is somewhat oxidized even under continuously flooded conditions, since oxygen is released through the root epidermal layer as a result of radial oxygen diffusion from the aerenchyma structure. The objective of this study was to compare the impacts of continuous vs. intermittent flooding on the speciation of As in the rice rhizosphere and grains, and relative abundances of microbial groups in rhizosphere compartments. We expected lower pore water As concentrations and eventually lower grain As concentrations in the intermittently flooded plots compared to the continuously flooded and also differences in As speciation between the water management practices.
Technical Abstract: Rice cultivated on arsenic (As) contaminated-soils usually accumulates variable grain-As concentrations, as impacted by genotypic differences, soil variables and crop management. A field-scale experiment was conducted to study the impact of intermittent and continuous flooding on As speciation and microbial populations in rice rhizosphere compartments of soils that were either historically amended with As pesticide or unamended with As. Rhizosphere-soil, root-plaque, pore-water and grain As were quantified and speciated, and microbial populations in rhizosphere soil and root-plaque were characterized. Total As concentrations in rhizosphere and grains were significantly lower in the intermittently flooded compared to the continuously flooded plots (86% lower in pore water, 55% lower in root-plaque and 41% lower in grain samples). iAsV, iAsIII and DMAsV were the dominant As species detected in rhizosphere-soil and root-plaque, pore-water and grain samples, respectively. Relative proportions of Archaea and iron-reducing bacteria (FeRB) were higher in rhizosphere soil compared to root-plaque. In rhizosphere soil, the relative abundance of FeRB was lower in intermittently flooded compared to continuously flooded plots, but there were no differences between root-plaque samples. This study has demonstrated that reductions in dissolved As concentrations in the rhizosphere and subsequent decreases in grain-As concentrations can be attained through water management.