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.