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

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Summary Methane (CH 4 ) produced in flooded paddy soils is emitted by passive diffusion, by ebullition and by diffusion through the rice ( Oryza sativa ) plant. Counter‐diffusing oxygen (O 2 ) 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 O 2 diffusion: increased cortical porosity, a reduced stelar radius, lower respiratory O 2 demand and subapical exodermal barriers to radial O 2 loss should widen the oxygenated rhizospheres in apical regions and reduce CH 4 emissions. This reduction arises because of increased rhizosphere resistance to CH 4 diffusion as the methanogenic front retreats rather than increased methanotrophic CH 4 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 CH 4 consumption, but resistance to CH 4 diffusion was sufficiently reduced for CH 4 emissions to increase. Overall, the results challenge the widely held assumption that increased methanotrophy will inevitably reduce CH 4 emissions. However, increased methanotrophy could reduce emissions where it replaces heterotrophic O 2 consumption.

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