Oxygenation Dynamics In The Rice Rhizosphere: Implications For Arsenic Uptake.

Arsenic contamination of rice grain is a global problem. Previous studies have suggested that the formation of iron-oxide plaque around rice roots — a result of atmospheric oxygen diffusing through hallow aerenchyma tissues in the plant out into anoxic rhizosphere soil — can limit plant uptake of arsenic by creating an in situ arsenic-adsorption surface. However, in soil systems, contention exists regarding the extent to which iron plaque formation reduces arsenic uptake by rice plants. Notably, iron plaque formation is spatially discontinuous along soil-grown roots, dependent on where and in what concentrations oxygen is released in the rhizosphere, and upon rates of other oxygen consuming processes (e.g., microbial respiration, methane oxidation). Planar optical oxygen sensors (optodes), which allow for real-time two-dimensional visualization of concentrations, provide an ideal tool for probing rhizosphere oxygenation dynamics. Here we present results from an experiment using oxygen optodes to visualize the extent, growth, and shrinkage of oxygenated zones in the rhizosphere of rice (Oryza sativa) grown in arsenic-contaminated soil under well-controlled experimental conditions. We use optode oxygen profiles to direct sampling of porewater and soil from mm-scale oxic and anoxic soil zones for key solutes and parameters to clarify the balance between oxygen consumption processes in the rhizosphere. These data, along with soil extractions and measurements of arsenic concentrations within plants help clarify the relationship between rhizosphere oxygen concentrations, iron-plaque formation and arsenic uptake by rice.