Arkansas rice rhizosphere and root plaques
Microbial communities and arsenic concentrations affected by water managements and soil arsenic levels in rice field
This field scale study was conducted to understand the impact of intermittent and continuous flooding systems on rhizosphere arsenic concentrations in relationship with rice-rhizosphere and root-plaque microbial populations. Samples were collected from plots experiencing continuous or intermittent flooding and having high or moderate soil arsenic concentrations. Community DNA was extracted and sequenced for 16S rRNA gene. The root-plaque communities from all 3-field replicates of each treatment were sequenced individually; whereas for the rhizosphere soil equal quantities of community DNA from all three-field replicates were composited into one sample per treatment.
Rice is grown primarily under continuously flooded conditions, which increases the accumulation of arsenic (As) in grain and consequently increases the risk of As exposure to consumers. Intermittent flooding could effectively reduce As accumulation in rice grain, but may also affect microbial populations in rice rhizosphere and thus potentially impact As chemistry and solubility. A field study comparing continuous and intermittent flooding in high- and low-As content soils was conducted to analyze treatment impacts on As concentrations and microbial populations in the rice rhizosphere and root-surface (root-plaque). Rhizosphere, root-plaque, and pore-water As concentrations were quantified using ICP-MS, and microbial populations in rhizosphere and root-plaque samples were characterized using quantitative-PCR and 16S rRNA gene sequencing. Pore-water As concentrations decreased by 81-86% in the intermittently flooded plots relative to the continuously flooded plots, with average pore-water As concentrations ranging from 3-37µg L-1. Bacteria dominated all treatments representing 91-94 % and 48-78% of the total community in root-plaque and rhizosphere, respectively, but Archaea and Fungi were relatively minor. The rhizosphere and root-plaque bacterial community composition differed significantly between the two soils with varied-As concentrations and flooding treatments. Proteobacteria was the predominant phylum in root-plaque (51-57 %) and most rhizosphere samples (23-27 %). The relative abundance of iron-reducing bacteria was lower in rhizosphere under intermittent than continuous flooding.   These results indicate that soil-As contents and water management systems can alter root-plaque and rhizosphere microbial communities, which may favorably contribute toward decreased dissolved-As concentrations in the rhizosphere of intermittently flooded plots relative to the continuously flooded treatment.