methane emission in ricefields

The atmosphere, by virtue of its chemical composition, is said to be the protective blanket of the earth against harmful radiation from the sun. It is composed primarily of nitrogen (78.08%), oxygen (20.95%), and argon (0.93%). When solar radiation strikes the Earth, energetic short wavelengths readily pass through the atmosphere and the Earth absorbs it and is warmed by it. The Earth, being a reflecting body, re-radiates this energy. However, this energy is being trapped by CO2, O3, CH4, and water vapor. In this process, these gases are viewed as beneficial since they maintain the global mean surface temperature of 15º C, thus keeping our natural life support system. This occurrence is known as the greenhouse effect with CO2, CH4, O3, and CFC’s labeled as greenhouse gases, GHG (Benarde, 1992).

However, energy balance is tilted once the concentration of these gases is not maintained at usual levels in the atmosphere. As mentioned, the capacity of the atmosphere to protect the Earth is based on its chemical composition. When the amount of these trace gases are increased, incoming solar radiation accumulates in the atmosphere beyond what is expected to ensure energy balance. Hence, global temperature is projected to rise unevenly causing global warming and climate change (Minami, 1994).

Together with CO2, gases such as CH4, N2O and CFC’s have shown large increases in their atmospheric concentration. As such, global warming may take place even more quickly than when only CO2 is taken into account. These radiatively active gases move from the troposphere to the stratosphere to participate in the depletion of the ozone layer (Minami, 1994). The modification of the chemical composition of the atmosphere is a result of human activities that have caused indirect impact on the Earth (Greene and Salt, 1993).

Rice cultivation is being pointed at as one of the four major sources of agricultural greenhouse gas emissions (Greene and Salt, 1993).



In the Philippines, rice is consumed at 21 500 megatons (MT) per day and comprises 40-80% daily calorie intake of the Filipinos (Ramos, 2000). As such, rice production must be kept at par with the growing demands of the population. Irrigated rice is grown on 2 M ha in which 1 M ha, 0.1 M ha and 0.4 ha are for rain-fed rice, tidal rice and upland rice, respectively. Therefore, there is a need to monitor the quantity of gas emissions from rice fields specifically in irrigated lowland. Identifying the levels of greenhouse gas emitted from rice cultivation would increase awareness on the extent of emission and would help scientists and policymakers come up with mitigation strategies as well as adaptation of the agriculture sector.