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REDUCTION OF RICEFIELD METHANE EMISSION

July 07, 1999

Methane (CH4) is considered to be the third most important gas, after carbon dioxide (CO2) and freons, in its contribution to the greenhouse effect and hence to global warming. Cores taken from the ice cap have shown that its concentration in the atmosphere has tripled in 100 years. This figure would explain about 20% of the rise in temperature (+1° C) observed in the biosphere during the century. Methane is highly absorbant of infrared, which gives it a warming capacity 20 times that of carbon dioxide. Moreover, it weakens the atmosphere's ability to oxidize tropospheric pollutants such as freons. Indirectly therefore it contributes to destruction of the ozone layer, Earth's natural barrier against unltraviolet and indispensible for life.
Atmospheric methane in fact comes largely from biological processes. It is generated by bacteria in anaerobic environments (i.e. devoid of oxygen) during the breakdown of organic matter. About 70% of methane emissions stem from human activity, especially farming. Domestic ruminants produce about 80 million tonnes/year and ricefields about 60 million tonnes, which is 20 to 40% of emissions. For one kilogram of rice produced 120 g of methane is evolved. Now, in order to cater for the needs of the world's population, the annual rice production must be increased by around 60% in 30 years; such a rise will generate more organic matter (rice roots and straw, organic fertilizers and so on) decomposing in the flooded soils of paddy fields. Emissions of methane are expected to increase significantly in these areas unless suitable farming methods are developed to counter them.
Ricefield methane emission results from two antagonistic but interrelated microbial activities. In anaerobic soils, methanogen bacteria produce methane and in the aerobic zones, where oxygen is available (in roots (rhizosphere) and oxidized soil-water interface), methanotrophs consume by oxidation up to 90% of the methane produced. It is the remaining methane unconsumed by the methanotrophs which goes into the atmosphere, through the aerenchyma of aquatic plants, often by diffusion and ebulltion.
The IRD scientists have studied the ecology of the microorganisms involved in methane emission from flooded ricefields with the aim of defining ways of reducing it. They took samples from 22 sites representing the different types of ricefield encountered throughout the world. Laboratory analyses were conducted to examine and quantify the microflora involved and determine the potential methanogenic and methanotrophic activities of these soils.
These investigations have shown that the methanogens are present in all ricefield soils and more or less constant proportions whatever their density. However, their density is not a reflection of the soil's potential for consuming or producing methane. It is rather the degree of availability of organic carbon in the soil which determines methane production and the availability of methane in the aerobic zones of soils which is the deciding factor for methanotrophy.
The laboratory analyses have shown that all the soils studied, when placed under appropriate conditions, have a higher methanotrophic than methanogenic potential, and therefore sufficient to consume all the methane produced in situ. The conditions that would encourage expression of the methanotrophic potential are in any case not found all together in constantly irrigated ricefields, where the soil is mainly anaerobic. Owing to this fact, only well-adapted farming practices can lead to any reduction in methane emission by the ricefields.
Tests in experimental plots of IRRI have thus shown that by draining a plot twice during one growing cycle, it is possible to cut methane emissions substantially. Results from laboratory investigations on the 22 soils studied indicates that this intermittent drainage will have the same effect in most of the ricefield soils.
A system of intermittent draining now appears to be the most realistic means of reducing significantly ricefield methane. Such a solution offers advantages for rice farmers, such as for the control of certain diseases of rice and predators, and also vectors of human diseases, like mosquitoes or certain molluscs, which develop in ricefield water.

FOR FURTHER INFORMATION

Contact : Pierre Roger or Jean Le Mer, IRD, Université de Provence, CESB/ESIL, Case 925, 163 Avenue de Luminy, F-13288, Marseille Cedex 9, Tél.: 33 (0)4 91 82 85 71, fax.: 33 (0)4 91 82 85 70, e-mail : rogerpa@orstom.esil.univ-mrs.fr

Bibliography
C. Joulian, S. Escoffier, J. Le Mer, H.-U. Neue, P.-A. Roger " Populations and potential activities of methanogens and methanotrophs in ricefields; relations with soil properties. " European Journal of Soil Biology , 1997, 33 (2).
P.-A. Roger, C. Joulian " Environmental impacts of rice cultivation ", in " Rice quality: a pluridisciplinary approach ", Cahiers Options Méditerranéennes, Vol. 24, n° 3- Article n° 38 (CD ROM), CIHEAM, 1998.
P.-A. Roger, J. Le Mer, C. Joulian, " L'émission de méthane par les sols : mécanismes, intensité et régulation ", Comptes Rendus de l'Académie d'Agriculture , 1999, sous presse.

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Institut de Recherche pour le Développement, Paris (IRD)