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

Could the alkaline soils of the world be the missing carbon sink?

The missing carbon sink is the large amount of unidentified carbon sink in the global carbon budget. According to the Woods Hole Research Center (2007) the average annual carbon emissions amount to 8.5 Pg (1 Pg or petagram is equal to 1 billion metric tonnes) comprising of 6.3 Pg from combustion of fossil fuels and 2.2 Pg from changes in land use. This is greater than the sum of the annual accumulation of carbon in the atmosphere (3.2 Pg) plus the annual uptake by the oceans (2.4 Pg) which is only 5.6 Pg. The difference of 2.9 Pg (i.e. 8.5-5.6=2.9) is unknown carbon sink required to balance the carbon budget.

Scientists have been searching for this big amount of unknown carbon sink during the last two decades. It was first thought to be located in the ocean considering that it occupies 70% of the earth’s surface. However, most scientists consider that the ocean sink is not big enough to account for the missing carbon (Xie et al., 2009). The next possible location is the world’s forest. In fact, many scientists believe that this large amount of missing carbon is absorbed by land-based carbon sinks particularly forests but estimates indicate that terrestrial ecosystems are a net sink of only 0.7 billion (Woods Hole Research Center, 2007). Some studies have revealed that carbon accumulation is largely counterbalanced by carbon loss from deforestation.

In a study published in Science, an international team of scientists led by Stephens (Stephens et al., 2007) revealed that the missing link may indeed be located in tropical ecosystems. They reported that northern terrestrial uptake of industrial carbon dioxide emissions is smaller than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for carbon dioxide. But whether or not this is enough to account for the missing carbon is not yet clear.

The third possible location of the missing carbon sink is the soil which is one of the largest dynamic carbon pools on earth. In a recent paper published in the journal Environmental Geology, researchers from China revealed a carbon sink which has been largely overlooked in the past. Xie et al. (2009) reported that alkaline soils (i.e. soils with pH > 7.0) on land are absorbing CO2 at a rate of 0.3-3.0 μmol m-2 s-1 with an inorganic, non-biological process. The intensity of this CO2 absorption is determined by the salinity, alkalinity, temperature and water content of the saline/alkaline soils. They estimated the range at 62-622 g C m-2 year-1. Considering that there are about 700 million hectares of alkaline soils around the world, the amount of CO2 absorption could be very significant on a global scale and could be a major part of the missing carbon sink.

References

Woods Hole Research Center. 2007. The missing carbon sink. http://www.whrc(carbon/missingc.htm

Stephens B.B. et al. 2007. Weak northern and strong tropical land carbon uptake from vertical profile of atmospheric CO2. Science 316: 1732-1735.

Xie J, Y Li, C Zhai, C Li and Z Lan. 2009. CO2 absorption by alkaline soils and its implication to the global carbon cycle. Environmental Geology 56: 953-961.