Source: Banker, B.C., H.K. Kludze, D.P. Alford, R.D. DeLaune, C.W. Lindau. 1995. Methane sources and sinks in paddy rice soils: relationship to emissions. Agric. Ecosys. Environ. 53:243-251.

Methane is a very important greenhouse gas. Greenhouse gasses trap heat energy in the atmosphere, creating a heating effect similar to a greenhouse. This may cause global warming. Methane is 30 times more effective than carbon dioxide as a greenhouse gas. Methane is increasing at the rate of 1% per year, which is close to 30 million metric tons. Rice paddies are the fourth largest contributor of methane to the atmosphere. Working at the Wetland Biogeochemistry Institute at Louisiana State University, the authors set out to determine methane emissions from paddy rice soils, from the root rhizospheres and from the entire paddy system. Total methane production from the entire paddy system was determined throughout the growing seasons of the first and second crop. This made it possible to determine times of sources and sinks of methane gas. Soil cores taken from between plots and the root rhizospheres were tested using methylflouride, a inhibitor of methane-oxidizing bacteria. This allowed the effect of the bacteria on methane emissions to be determined. For the entire paddy they found emissions from the second crop were much higher than from the first crop. Methane emissions were variable over the growing season, with the lowest rate measured during the first stages and the highest rate measured during the middle stages. This was the case for both crops. The soil emitted small amounts of methane, but the difference between methylflouride induced soils and the control indicated that bacteria had a significant effect in lowering methane levels in the soil. Similar effects were noted in the plant rhizosphere. The greatest methane emissions came from the methylflouride treated rhizospheres. The methane oxidation taking place in the root flasks compared to the fallow flasks was threefold greater. This led them to the conclusion that 95% of methane emitted to the atmosphere passes through the plant but "methane oxidation in the surface oxidized soil and in the root rhizosphere significantly reduces the amount of atmospheric methane emissions from the Louisiana paddy soils." This study also showed that even though there are great amounts of methane emitted from paddy rice systems, there are times when methane is being oxidized at a greater rate than it is being emitted. If it is possible to learn how this happens and how this could be sustained, it could mean a tremendous reduction of methane in the atmosphere. If global warming is taking place, then the reduction of methane would slow the greenhouse effect and help stabilize atmospheric temperature.

Abstract author: Drew Smith, 26 October 1995.

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