Somenahally, Anil Kumar C. (2010-12). Impact of Two Water Management Systems on Arsenic Speciation and Microbial Populations in Rice Rhizosphere. Doctoral Dissertation. Thesis uri icon

abstract

  • Arsenic (As) is a problem with rice production systems throughout the world as high As concentrations are reported in rice grains originating from several parts of the world. This characteristic is mainly due to the flooded conditions utilized in rice culture. We hypothesized that the soluble As concentrations in the rice rhizosphere can be decreased by growing rice more aerobically through intermittent flooding. Intermittent water management practices might also change microbial populations in the rice rhizosphere that might potentially impact As chemistry and bioavailability. Two field-scale experiments were conducted over two years to study the impact of intermittent and continuous flooding on As speciation and microbial populations in the rice rhizosphere. As levels and speciation in the rhizosphere soil, root-plaque and pore-water were determined using a high performance liquid chromatography-inductively coupled plasmamass spectroscopy (HPLC-ICP-MS). The microbial populations were assessed from the rhizosphere soil and root-plaque samples using quantitative polymerase chain reaction (qPCR) and 16S rRNA sequencing. Pore-water and root-plaque total-As concentrations significantly decreased in the intermittent compared to the continuous flood plots. Inorganic arsenite (iAsIII) was predominant in pore-water and inorganic arsenate (iAsV) in root-plaque and soil. Rootplaque sequestered significantly higher levels of As (almost tenfold higher) than the adjacent rhizosphere soil. Grain As concentrations also decreased by 35 to 45 percent in the intermittent compared to the continuously flooded plots. Organic As species, monomethyl and dimethyl arsenate were detected in the rhizosphere with relative increases and decreases among the treatments. Bacteria were the predominant group (91 to 94 percent and 48 to 78 percent of total community in root-plaque and rhizosphere soils, respectively). Archaea were also a major component of rhizosphere soil with their populations being higher under continuous flooding. The relative abundance of iron-reducing bacteria was around 3 to 6 percent of the total community in root-plaque and around 6 to 6 percent in soil, with significantly lower abundance in the intermittent compared to the continuously flooded plots. Results of these studies demonstrated that intermittent flooding could be a potential management option to reduce grain As in rice cultivated on fields with moderate to high As concentrations.
  • Arsenic (As) is a problem with rice production systems throughout the world as high As

    concentrations are reported in rice grains originating from several parts of the world. This

    characteristic is mainly due to the flooded conditions utilized in rice culture. We hypothesized

    that the soluble As concentrations in the rice rhizosphere can be decreased by growing rice more

    aerobically through intermittent flooding. Intermittent water management practices might also

    change microbial populations in the rice rhizosphere that might potentially impact As chemistry

    and bioavailability. Two field-scale experiments were conducted over two years to study the

    impact of intermittent and continuous flooding on As speciation and microbial populations in the

    rice rhizosphere. As levels and speciation in the rhizosphere soil, root-plaque and pore-water

    were determined using a high performance liquid chromatography-inductively coupled plasmamass

    spectroscopy (HPLC-ICP-MS). The microbial populations were assessed from the

    rhizosphere soil and root-plaque samples using quantitative polymerase chain reaction (qPCR)

    and 16S rRNA sequencing. Pore-water and root-plaque total-As concentrations significantly

    decreased in the intermittent compared to the continuous flood plots. Inorganic arsenite (iAsIII)

    was predominant in pore-water and inorganic arsenate (iAsV) in root-plaque and soil. Rootplaque sequestered significantly higher levels of As (almost tenfold higher) than the adjacent

    rhizosphere soil. Grain As concentrations also decreased by 35 to 45 percent in the intermittent

    compared to the continuously flooded plots. Organic As species, monomethyl and dimethyl

    arsenate were detected in the rhizosphere with relative increases and decreases among the

    treatments. Bacteria were the predominant group (91 to 94 percent and 48 to 78 percent of total community

    in root-plaque and rhizosphere soils, respectively). Archaea were also a major component of

    rhizosphere soil with their populations being higher under continuous flooding. The relative

    abundance of iron-reducing bacteria was around 3 to 6 percent of the total community in root-plaque

    and around 6 to 6 percent in soil, with significantly lower abundance in the intermittent compared to

    the continuously flooded plots. Results of these studies demonstrated that intermittent flooding

    could be a potential management option to reduce grain As in rice cultivated on fields with

    moderate to high As concentrations.

publication date

  • December 2010