Methane oxidation

Methane is the third most important greenhouse gas, after CO2 and H2O. Moreover, takes the second place in the group of greenhouse gases which are seriously influenced by the anthropogenic activities. Its concentration has risen from 0.7 ppm in the pre-industrial period to 1.7 ppm, at present it is responsible for 20% of the radiative forcing. The contribution of rice paddies to the total emission of methane (530 Tg per year) is considerable but not known precisely. This uncertainty is partially caused by the large variations in local rice growth conditions and by the complicated dynamics between the methane production and methane oxidation in the rice paddy soil. Therefore, a better understanding of methane oxidation in the rice rhizosphere is necessary.

Under anaerobic conditions, methanogenic bacteria produce methane in the paddy soil. Their production depends highly upon the availability of degradable organic matter. Methanogens use acetate and H2/CO2 derived from organic material as substrates. Methane production is fuelled by exudates of the roots and is highest at the end of the growing season when the roots are completely developed. The start of methane production in wetland rice varies from a few days to several weeks after flooding the field, depending on the chemical and microbial conditions of the soil. Before flooding, wetland rice fields contain similarly high numbers of viable methanogenic bacteria as under anaerobic conditions. Thus, the onset of CH4 production apparently does not depend on the growth in number of methanogenic bacteria.

Methane can escape from the rice paddy soil in various ways: via aerenchyma in the plant (90%), via ebullition (10%), or via diffusion through the soil and water layer (1%). Ebullition dominates in unvegetated plots. Methane transport via the plant starts in the roots; methane enters by diffusion through the epidermis and during the water uptake. It is likely that dissolved CH4 is directly gasified in the root cortex and further diffuses upwards to the root-shoot transition zone via intercellular spaces and aerenchyma. The aerenchyma system is developed by the plant to transport the oxygen necessary for respiration towards the roots. Just like methane diffuses from the soil into the root system, oxygen diffuses from the root into the soil, creating a relative oxygen rich zone in the rhizosphere. Methane produced by methanogenic bacteria in the soil is partly oxidized in the rhizosphere to CO2 by methanotrophic bacteria. Methanogenesis in the rhizosphere itself is suppressed by oxygen (more).