Chapter 25 Greenhouse Gases Emissions from Natural Systems: Mechanisms and Control Strategies




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CHAPTER 25






Greenhouse Gases Emissions from Natural Systems: Mechanisms and Control Strategies




Xiaolei Zhang, Song Yan, R. D. Tyagi, Rao Y. Surampalli, and

Tian C. Zhang




25.1 Introduction



Greenhouse gas (GHG) is emitted from human activities and natural systems. The former is counted as the major source of the emission (around 70% of total emissions); however, the latter also pronounce a great amount of emission, which is around 4800 TgCO2 equivalent per year (U.S. EPA 2010). The GHG emissions from natural systems majorly include the emissions from wetlands, oceans, freshwater bodies, permafrost, termites, ruminant animals, geologic settings, and wildfire, and among all, wetlands are majorly responsible (Song et al. 2008; Danevčič et al. 2010).


Wetlands are divided into peat wetlands, also called peat lands, and non-peat wetlands (Wilson et al. 2001; Blain et al. 2006). GHGs emitted from wetlands are mainly in the form of methane rather than carbon dioxide and nitrous oxide. Reports have shown that wetlands are one of the primary sources of atmospheric methane, which accounts for 3900 TgCO2 equivalent per year (> 81% of total natural system GHG emissions) (Zhuang et al. 2009; U.S. EPA 2010). The GHG emissions are due to the degradation of organic materials under the anoxic condition. Strategies that are to cut off methane production or diffusion to the atmosphere should be developed for controlling the emissions from wetlands and peat lands. It is known that methane production is due to the domination of methanogenic microorganisms in the system; therefore, it would mitigate methane emission by promoting the growth of methanotrophs and other microbial communities to diminish the growth of methanogens. When the production occurs, capturing and storing it before it enters into the atmosphere would also be a method (Bourrelly et al. 2005; Chathoth et al. 2010).


Compared to wetlands, other natural systems (oceans, freshwater bodies, permafrost, termites, ruminant animals, geologic settings, and wildfire) contribute a small fraction of the GHG emissions from all natural systems (< 20% of total). The emission from oceans and freshwater are not well understood; however, it may be linked with two important factors: a) the result of anaerobic digestion of fish and zooplankton and b) the result of methanogenic microorganism activities in the sediments (Levitt 2011). Billions of tonnes of methane are locked on the Arctic soil, while as permafrost melts, there is a great risk of methane seeping. In fact, it is a vicious circle because as methane emission increases, thawing of permafrost would be enhanced, which would result in more methane emission (Laurion et al. 2010). Termites is considered as the second largest methane emission natural sources (the first largest is wetlands and peat lands). Methane is produced in their normal digestion process, and the production amount varies according to the species and regions. Ruminant animals such as cattle, sheep, and wild animals are methane emission sources as well. The emission is mainly from the digestion, and highly depending in the population of animals. Geothermal-volcanic systems and hydrocarbon-generation processes in sedimentary basins are two major sources of GHG geologic emissions. These emissions have always been neglected or paid little attentions before year of 2000, while over the last ten years studies have been done to confirm that geological GHG emission significantly contributes to the global GHG emission (Etiope and Klusman 2002; Etiope 2009). Wildfires, also called natural forest fires, also causes GHG emission including carbon dioxide and methane, because of incomplete combustion of organic material.


In this chapter, the mechanisms of GHG emissions from natural systems including wetlands, oceans, freshwater, etc., are described; the strategies to control GHG emissions are discussed.


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