Methane Emission From Flooded Soils - From Microorganisms to the Atmosphere.

Flooded soils such as rice fields and wetlands are the most important source for the greenhouse gas methane. Rice fields, in particular, serve as model for studying the role of the structure of anaerobic microbial communities for ecosystem functioning and the partitioning of carbon flux along different paths of degradation of organic matter to methane. Flooded soils are relatively rapidly depleted of oxygen and other oxidants such as ferric iron and sulfate. Then, organic matter degradation results in the production methane. Methane is eventually produced from different types of organic matter, mainly from plant litter, root exudates, and soil organic matter. Methane production is achieved by a community consisting of hydrolytic, fermenting and methanogenic microorganisms. Acetate and hydrogen (plus CO₂) are the two most important fermentation products that are used as methanogenic substrates to different extent. The transport of CH₄ to the atmosphere is mainly partitioned between transport through the aerenchyma system of plants, gas ebullition and diffusion. Transport through oxygenated zones such as the surface soil or the rhizosphere results in oxidation of a substantial percentage of methane by methanotrophic bacteria thus attenuating the methane flux into the atmosphere. The system thus offers different ecological niches for microorganisms and different scales of activity that eventually all contribute to the emission of methane into the atmosphere.