Methane emission from rice fields

Methane (CH4) and carbon dioxide (CO2) are the end products of carbon decomposition in rice fields and other wetlands. Methane, a major greenhouse gas, is the terminal step of the anaerobic breakdown of organic matter in wetland soils. It is exclusively produced by methanogenic bacteria that can metabolize only in the absence of free oxygen and at redox potentials below -150 mV (Neue et al. 1997).

According to the above-cited paper by Dr.H.U. Neue (former Head of the Soils Department at IRRI and later Professor of Soil Chemistry at the University of Halle-Wittenberg, Germany) one of the pioneers in methane research in rice fields, methane is largely produced by transmethylation of acetic acid and to some extent, by the reduction of carbon dioxide in wetland soils.

The rate and pattern of organic matter addition and decomposition also contribute to the rate and pattern of methane production. In rice field, methane production generally increases during the cropping season. Easily degradable soil carbon, plant litter, root exudates, decomposing roots and aquatic biomass that are added to the anaerobic zone of the paddy soil (this is the zone below the thin oxidized or brown soil surface) are the major carbon sources for methane production.

Presently, there is widespread research interest in the development of methods and strategies to reduce methane emission from rice fields and other wetlands. Some early studies have shown that sodium chloride at high concentration inhibits methane formation. Addition of sea water has also been found to inhibit methane formation at low salt concentration because of its sulfate content. Very recently, Dr. Roel R. Suralta and colleagues at Philrice, Nueva Ecija, have demonstrated that iron fertilizer application significantly reduced methane emission from rice field. More importantly, the iron fertilizer application also increased rice yield (Suralta et al., 2011).

References

Neue HU, JL Gaunt, ZP Wang, P Becker-Heidmann, and C Quijano. 1997. Carbon in tropical wetlands. Geoderma 79: 163-185.

Suralta RR, FS Gorospe, CA Asis Jr and K Inubushi. 2011. Effect of iron fertilizer application on the yield and methane emission of paddy rice field. In: Proceedings of the 14th Annual Meeting and Scientific Conference, Philippine Society of Soil Science and Technology (PSSST), VSU, Baybay, Leyte 25-27 May 2011, pp:95-96