Visualization of Oxygenation Dynamics in the Rice Rhizosphere Under Elevated CO2 and Temperature.

In the anoxic soil zone around rice roots (the rhizosphere), microbial respiration, iron oxidation, and methane oxidation all compete for oxygen that diffuses from the atmosphere, through aerenchyma tissue in the plant, out into the rhizosphere soil; competition between these three reactions can impact rice field methane emissions and nutritional quality of rice grains. The balance between these oxygen-consumption reactions is complex, dependent upon chemical, physical, and biological processes occurring at micrometer scales. Such complexity at small scales has limited our ability to ascertain which are the key concentrations and relationships controlling the competitive balance, and has made it difficult to predict how the competitive balance may shift in the future as temperature and atmospheric CO₂ concentrations change.

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 initial experiments using oxygen optodes to visualize the extent, growth, and shrinkage of oxygenated zones in the rhizosphere of rice (Oryza sativa) grown in natural soil under well-controlled experimental conditions where soil temperature and atmospheric CO₂ concentrations are varied. The goal is to 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 and determine their respective responses to climate conditions so that we can ultimately assess how rice field methane emissions or rice grain nutritional quality may change in the future.