Microbial communities in rice rhizosphere altered by intermittent and continuous flooding in fields with long-term arsenic application

Authors: SOMENAHALLY, ANIL - Texas A&M University; HOLLISTER, EMILY - Texas A&M University; LOEPPERT, RICHARD - Texas A&M University; Yan, Wengui; GENTRY, TERRY - Texas A&M University

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2011
Publication Date: 2/26/2011
Citation: Somenahally, A., Hollister, E.B., Loeppert, R.H., Yan, W., Gentry, T.J. 2011. Microbial communities in rice rhizosphere altered by intermittent and continuous flooding in fields with long-term arsenic application. Soil Biology and Biochemistry. 43(2011):1220-1228.

Interpretive Summary: Arsenic is toxic to most forms of life including microorganisms. Arsenic occurs naturally in soils and can also be found as a result of the use some agricultural chemicals. More than 50-million people in the world are exposed to high arsenic concentrations in drinking water as a result of naturally high levels of arsenic in some soils. Rice can accumulate higher amounts of arsenic than other cereals, largely due to the arsenic being more available for plant uptake in the anaerobic flooded soils commonly used in rice production. Arsenic exposure through the consumption of high arsenic containing rice grain is an important exposure route for a sizeable portion of the world’s population. Studies are needed to understand the impacts of different water-management practices on soil-microbial communities in order to ultimately understand the role of microorganisms in controlling arsenic biogeochemistry and bioavailability. We conducted a field experiment to investigate changes in rice-rhizosphere arsenic concentration and microbial communities during the rice growing season using continuous or intermittent flooding, as well as high and low arsenic containing soils. We found that water saving irrigation practices may reduce the amount of arsenic available for uptake by rice plants. Thus, limiting the amount of arsenic in the grain.

Technical Abstract: Rice cultivated on arsenic -contaminated soils can, under some conditions, accumulate high concentrations in the grain, particularly continuous flooding practices are used. Intermittent flooding, as opposed to continuous flooding, could be a means to reduce soluble arsenic concentrations in the rice rhizosphere and alter soil microbial populations that may impact arsenic availability. A field-scale study was conducted to analyze arsenic concentrations and microbial populations in the rice-rhizosphere, in response to different irrigation practices and different soil arsenic levels. Rhizosphere-soil and pore-water arsenic concentrations were quantified using an inductively-coupled-plasma mass spectrometer, while microbial populations in the rice-rhizosphere soil were characterized using community qPCR and 16S rRNA gene sequencing. Average pore-water arsenic concentrations decreased 48 % from 44 µg L-1 in the continuously flooded plots to 17 µg L-1 in the intermittently flooded plots. The relative abundance of bacteria increased over the course of the growing season, while archaeal and fungal gene abundances decreased. Two irrigation treatments brought about a significant difference in bacterial community composition. The relative abundance of iron-reducing bacteria and sulfate-reducing bacteria were significantly higher under the continuous flood than the intermittent flood treatment, implying greater relative iron reduction and possibly arsenic release from the iron oxides under the continuous flood conditions. These results indicate that rhizosphere microbial populations were different between the intermittent flood and the continuous flood treatments, which may be contributable to the lower pore-water arsenic concentrations in the intermittently flooded conditions. These results indicate that water saving irrigation management may be a means to reduce the amount of arsenic available for uptake by rice plants.