Rice paddies provide staple food for nearly half the world population, while they represent one of the main anthropogenic sources of atmospheric CH₄, a potent greenhouse gas. Toward an ambitious goal of 80% reduction of rice-paddy CH₄ emissions, we will make two new technologies that can: (i) modify rice genotypes to have more aerobic rhizosphere where active CH₄ oxidation takes place by soil methanotrophs, and (ii) maximize activity of CH₄-oxidizing N-fixing endophytic bacteria inhabiting inside the rice plants so that CH₄ is utilized as an energy source for N fixation.
Rice paddies represents one of the largest anthropogenic sources of atmospheric CH₄, the second largest driver of global warming after CO₂. Technologies that can slash CH₄ emissions are urgently needed without sacrificing grain yield of rice. Methane is produced by microbes in environments where oxygen is limited. Because of the ponding water, paddy soil becomes anoxic, offering favorable condition for active methanogenesis through degradation of organic matters (straw, dead roots). Typically, 3-7% of the fixed carbon by rice plants is converted to CH₄. These numbers appear to be small, but rice paddy contributes global warming substantially because CH₄ is 28-36 times more potent than CO₂ at trapping heat over a 100-year time frame.
CH₄ emission from rice paddies is determined by the balance between CH₄ production and CH₄ oxidation. Although paddy soil is basically anaerobic, living root and surrounding soil (rhizosphere) can be aerobic, because rice plants develop tissues to transfer oxygen to the belowground (aerenchyma). Active oxidation of CH₄ by methane-oxidizing bacteria (methanotrophs) occurs in rice rhizosphere.
In this Moonshot project, we will develop holistic technologies that can maximize the activity of the methanotrophs so that produced CH₄ in anaerobic part of soil could be oxidized to CO₂ before reaching the atmosphere. Because organic matters originate from atmospheric CO₂, overall greenhouse effect would be null when CH₄ is converted to CO₂ (carbon neutral).
In Theme IV-1, we will develop technologies for enhanced CH₄ oxidation in rice rhizosphere. Root traits that help creating more aerobic rhizosphere will be identified and major rice varieties will be genetically improved to accommodate such functional traits that promote activity of methanotrophs.
In Theme IV-2, we will realize simultaneous stimulation of CH₄ oxidation and nitrogen fixation conducted by specific microbes living inside the rice tissue. Our research team (Tohoku university) have discovered a methanotroph that also has an ability to fix atmospheric nitrogen inside the tissue of rice root. In this project, we will unveil the symbiotic mechanisms and develop technologies to maximize the activity of the CH₄-oxidizing N₂-fixing bacteria. With these technologies, CH₄ emissions and amount of nitrogen fertilizer will be reduced simultaneously.
Photo: Evaluating CH₄ emissions from various rice genotypes by a closed-chamber method.