**2. Rice cultivation, methane cycling and global warming**

Global warming is a serious problem nowadays. CH4 is one of the vital greenhouse gases that contribute to global warming. About 60% of global CH4 emissions are occurred because of anthropogenic activities [17]. The main sources of anthropogenic CH4 emissions are the oil and gas industries, agriculture, landfills, wastewater treatment, and emissions from coal mines. Globally, about 530 million tons of CH4 (converted in terms of carbon) are emitted annually [17]. Rice fields are contributing to global warming, but it is a far bigger problem than previously thought. The conventional paddy field with continuous flooding irrigation is known as a major source of CH4 emission [18]. The world's largest rice producers are China and India. After China and India, the next largest rice producers are Indonesia, Bangladesh, Vietnam, Myanmar, and Thailand. It is expected that economic activities will become more active in the next future mainly in Asia. Reportedly, of the Asian population of about 4200 million people, about 2700 million (60%) live on rice. Since Asia's population is expected to continue increasing, the total area of paddy fields in Asia will also increase. The paddy area in 2009 was 10,940,000 ha in Thailand, 4,410,000 ha in the Philippines, 7,420,000 ha in Vietnam and 12,100,000 ha in Indonesia [19]. So a hues amount of population needs more rice, more rice growing mean more CH4 adding to the atmosphere. During 1 kg of rice grain production, paddy field contributes 100 g of CH4 to the atmosphere. The default methane baseline emission factor is 1.3 kg CH4 ha−1−day−1, in continuous flooding rice cultivation [20, 21]. Part of the CH4 produced in the rice soil is consumed in the oxidized rhizosphere of rice roots or in the oxidized soil-floodwater interface. Soil bacteria also can consume CH4 [22]. CH4 is also leached to ground water, as a small part dissolves in water and most of it escapes from the soil into the atmosphere (**see Figure 1**). The important thing is most of the CH4 is staying in the atmosphere for 10 years as it is or the increasing CH4 abundance leads to a longer lifetime for CH4 [23]. After a certain time later, the CH4 is broken down into CO2. Approximately 95% of the CH4 produced in flooded soils is oxidized to CO2 before it release to the environment [24]. It is still not clear exactly how much of the CH4 is finally converted to CO2 and how much might remain as other intermediate carbon-containing compounds without a significant direct effect on the climate [25]. However, CH4 initially reacts with ozone in a 'chain' reaction that ultimately produces CO2 and water vapor. CH4 also creates ground-level ozone in the atmosphere. And ozone is not only harmful to human health but also contributes to climate change. CH4 is much more effective than CO2 at trapping heat (more than 60%) in the atmosphere [26]. Global temperatures in 2014 and 2015 were warmer than at any other time in the modern temperature record after 1880. And carbon emissions are the central cause of that rise. Rising temperatures and changes in rainfall have a significant effect on enhancing microbial activity and create ideal conditions for microbial CH4

**73**

the world.

**Figure 1.**

**3. Methane chemistry**

*Reactions of Methane*

CH4 + H2O, 850o

1.Combustion (oxidation)

*Methane Cycling in Paddy Field: A Global Warming Issue*

production of flooded rice fields. It is predicted that atmospheric temperature will rise 2°C by 2060. If it is actually going to happen, most of the agricultural plants cannot grow; it will make severe starvation for the world population. On the other hand, sea level will rise. The consequences will be so devastating, most of the low lands will go underneath the water and it will increase homeless people all over

*Biogeochemical mechanisms of CH4 production of paddy field and it's cycling (modify from [15]).*

CH4 is a very special kind of molecule. CH4 is an end product of the organic carbon

decomposition under anoxic conditions and the simplest organic compound and member of the paraffin series of hydrocarbons [27]. It is colorless, odorless gas that occurs abundantly in nature and as a product of anthropogenic activities. Its chemical formula is CH4 (**Figure 2**). CH4 is lighter than air, having a specific gravity of 0.554. It is slightly soluble gas in water and burns readily in air, forming carbon dioxide and water vapor; the flame is pale, luminous and very hot. The boiling point of CH4 is −162°C and the melting point is −182.5°C. Basically, CH4 is very stable, but mixtures of CH4 and air,

a.Complete oxidation: CH4 + 2 O2, flame or spark → CO2 + H2O + heat

with the content between 5 and 14% by volume, are explosive [28].

b.Partial oxidation: 6 CH4 + O2, 1500°C → CO + H2 + H2C2

, Ni → CO + H2

*DOI: http://dx.doi.org/10.5772/intechopen.94200*

*Methane Cycling in Paddy Field: A Global Warming Issue DOI: http://dx.doi.org/10.5772/intechopen.94200*

**Figure 1.** *Biogeochemical mechanisms of CH4 production of paddy field and it's cycling (modify from [15]).*

production of flooded rice fields. It is predicted that atmospheric temperature will rise 2°C by 2060. If it is actually going to happen, most of the agricultural plants cannot grow; it will make severe starvation for the world population. On the other hand, sea level will rise. The consequences will be so devastating, most of the low lands will go underneath the water and it will increase homeless people all over the world.
