Microbial Community Response to Two Water Management Systems for Wetland Rice Production in High Arsenic Soils.

Rice cultivation on As-impacted soils has recently garnered considerable attention.  Rice plants can accumulate As in grain, especially under the continuously flooded conditions commonly utilized in wetland-rice production. However, recent studies have indicated that rice-management systems that utilize less water, e.g, intermittent flooding, and favor a less reduced rooting zone will result in lower rice-grain As concentrations. This reduction in grain-As concentration might be due to changes in the populations of specific soil microorganisms that can impact As chemistry through oxidation/reduction and methylation/demethylation processes. The objectives of the current study were to evaluate the impact of continuous and intermittent flooding of rice paddies on soil microbial community structure and As speciation in the rice rhizosphere.  A field experiment was conducted near Stuttgart, AR  (2007 and 2008), with two soil As levels (native paddy and addition of monosodium methanearsonate [MSMA]) and two water-management practices (continuous and intermittent flooding). Rhizosphere soil and pore-water samples were collected at several times during the rice-growing season. Organic- and inorganic-As species in pore-water samples were determined by HPLC-ICP-MS. The relative abundances of total bacteria:fungi:archaea and specific microbial groups including Geobacter spp., sulfate reducing bacteria, Shewanella spp. and methanogens in the rhizosphere soil were enumerated by qPCR. The archaea:bacteria ratios increased while fungi:bacteria ratios decreased after flooding the soils. High As concentrations resulted in decreased fungal populations, whereas flooding resulted in increased archaea and decreased fungi populations. At the end of the rice-growing season, bacterial as well as methanogen populations dominated in most of the treatment plots.  The methanogens were significantly higher in continuously flooded plots than in intermittently flooded plots. However, this trend was not observed for other anaerobes such as sulfate-reducing prokaryotes and Geobacter spp., which showed no significant differences with water treatment. Inorganic- and organic-As species were present in significantly higher concentrations in the continuous flood treatments, with As^III being the predominant form in pore-water samples. Methyl-As compounds, including dimethylarsinic acid, were only detected in the high As and continuously flooded plots, indicating microbial methylation and demethylation of As in these systems. The results suggest that the rhizosphere microbial community changed under different water management practices, which likely contributed to differences in As speciation and uptake by the rice plants.