Importance of rice root oxidation potential as a regulator of CH4 production under waterlogged conditions

One of the most important characteristics of a rice cultivar controlling methane (CH4) production can be the root oxidation potential because a cultivar with a high oxygen (O-2)-releasing capacity can create an oxidized root area in the rhizosphere and this can oxidize CH4 during rice cultivation. The root oxidation potential of six Korean rice cultivars grown in a minirhizotron was measured by digital image analysis of the oxidized root areas and compared with the conventional method (root oxidase activity by using alpha-napthylamine as a substrate). In addition, pmoA gene copy numbers of the rhizosphere of the cultivars, indicating soil methanotrophic bacterial population and measured by qPCR assays, were compared. Oxidized root area and pmoA gene copy numbers differed significantly among cultivars (P < 0.05), but not the root oxidase activity. Oxidized root area was significantly and negatively correlated with rhizospheric dissolved CH4-C and mean CH4 emission rate by plants and significantly and positively correlated with the rhizospheric dissolved carbon dioxide (CO2)-C and pmoA gene copy number as well as with some of selected plant growth parameters such as root biomass and root volume. The Dongjin cultivar had a high root oxidation potential, while Nampyeong, Chuchung, and Samkwang had a low root oxidation potential under a flooded paddy soil environment. In addition, the characterization of oxidized root area by digital image analysis rather than the alpha-napthylamine oxidation method was more effective in differentiating root oxidation potential among different lowland rice cultivars.