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

Rice cultivated on As-contaminated soils can under some conditions, accumulate high concentrations of As in grain, mostly as a result of the continuous flooding practices commonly used for rice cultivation. Intermittent flooding, as opposed to continuous flooding, might reduce soluble As concentrations in the rice rhizosphere; however might also likely alter soil microbial populations that may impact As chemistry. A field-scale study was conducted to analyze As concentrations and microbial populations in the rice rhizosphere, in response to continuous and intermittent flooding practices under two levels of soil As. Rhizosphere-soil and pore-water As 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 As concentrations ranged from 44 µg L-1 in the continuously flooded plots to 17 µg L-1 in the intermittently flooded plots, representing a decrease of approximately 48 %. The relative abundance of Bacteria increased over the course of the growing season, while archaeal and fungal gene abundances decreased. The bacterial community composition was significantly different between the treatments. The relative abundance of iron-reducing bacteria and sulfate-reducing bacteria were significantly higher under the continuous flood relative to the intermittent flood treatment, implying greater relative iron reduction and possibly As release from the iron oxides with the continuous flood treatment. These results indicate that rhizosphere microbial populations were different in the intermittent flood compared to the continuously flooded plots, which may have contributed to the lower pore-water arsenic concentrations in the intermittently flooded plots.