Climate change and wetland rice ecosystem

Flooded ricefields are important sources of atmospheric methane on a global scale. At present ricefields contribute an estimated 10-20 percent of total atmospheric emission of 500 Tg. Bacterial decomposition of organic matter in flooded rice soils produces methane. The aerenchyma and intracellular space of rice plants mediate the transport of CH4 from the reduced soil to the atmosphere. Methane is released to the atmosphere through diffusion of dissolve methane, ebullition of gas bubbles and via the rice plant. Rice plants act as chimney for up to 90 percent of the methane that escape from flooded fields. Large portions of CH4 formed in the anaerobic soil may remain entrapped in the soil and is oxidized to CO2 after the soil falls dry. Field and laboratory studies show readily metabolizable carbon, soil reduction, soil pH, soil structure and texture, soil temperature, rice cultivars, and cultural practices are the major factors that control methane fluxes. At IRRI, measurements of methane emission are conducted with an automatic measurement system. Emission rates are highly variable. A distinct seasonal pattern is observed. Methane emission increases during the vegetable phase sometimes with an intermediate peak and continues to increase until it declines at the end of the season. Organic amendments directly increase methane production and emission. The interaction of methane formation, methane oxidation, rice growth and cultivation is complex. The required increase in rice production will increase methane emissions from ricefields if current technologies are kept. Research has begun to identify the mechanisms of methane fluxes in ricefields that open up the development of technically and socioeconomically feasible mitigation technologies without sacrificing rice yields. The most promising non regret options are modified water and soil management minimizing inputs of easily degradable carbon sources, and selecting rice with lower emission potential