Radial oxygen loss, rhizospheric methane oxidation and methane emissions from rice roots

Methane (CH₄) produced in flooded paddy soils is emitted by passive diffusion, by ebullition and by diffusion through the rice (Oryza sativa) plant. Counter-diffusing oxygen (O₂) via shoot to root and rhizosphere will inhibit methanogenesis and support aerobic rhizospheric methanotrophs and heterotrophs. In this study, we analyzed the anatomical, morphological and physiological traits of roots of several rice varieties and used modeling approaches to assess the potential effects on methane emissions. Modeling showed that traits that enhance internal O₂ diffusion: increased cortical porosity, a reduced stelar radius, lower respiratory O₂ demand and subapical exodermal barriers to radial O₂ loss should widen the oxygenated rhizospheres in apical regions and reduce CH₄ emissions. This reduction arises because of increased rhizosphere resistance to CH₄ diffusion as the methanogenic front retreats rather than increased methanotrophic CH₄ consumption. Increasing methanotrophic rate per unit volume raised emissions by narrowing the oxygenated rhizosphere. Larger root radii (with stelar radius constant) increased oxygenated rhizosphere thickness and CH₄ consumption, but resistance to CH₄ diffusion was sufficiently reduced for CH₄ emissions to increase. Overall, the results challenge the widely held assumption that increased methanotrophy will inevitably reduce CH₄ emissions. However, increased methanotrophy could reduce emissions where it replaces heterotrophic O₂ consumption.