Methane emission and rice agriculture

摘要

A great deal of evidence has accumulated showing that rice agriculture resulted in an increased emission of methane to the atmosphere. The reasons for the interest in methane are that (i) CH_4 is an important energy source, representing a clean and potentially economic alternative fuel, (ii) CH_4 has a global warming potential of about 24.5 relative to CO_2, and is responsible for approximately 25 percent of the anticipated warming. The mixing ratio of CH4 has been increasing and has reached a level of 1.8 ppmv in the atmosphere3. Over the past 20 years, CH4 growth rate has declined; in the late 1970s, the concentration was increasing by about 20 ppbv yr~(-1), and during the 1980s, the growth rate dropped to 9-13 ppbv yr~(-1). Around the middle of 1992, CH4 con centration briefly stopped growing but since 1993, the global growth rate has returned to about 8 ppbv per year. Dlugokencky et aLs also reported that the rate of increase of methane concentration has slowed down in the last decade. Atmospheric CH_4 originates mainly from biogenic sources, such as rice paddies and natural wetlands. The rice paddies account for 15-20 percent of the world's total anthropogenic CH_4 emission. Mechanistically, methane produced by methanogenesis in the soil (an energy-transformation process, mediated by methanogens at a soil redox potential lower than -140 to -160 mV) is transported to the atmosphere by molecular diffusion, ebullition or plant-mediated transport. More than 90 percent of methane released from rice soil to the atmosphere is emitted via the rice plant. Well-developed intercellular air spaces in leaf blades, leaf sheaths, culm and roots of rice plant provide an efficient gas-exchange medium between the atmosphere and the anaerobic soil. CH_4 dissolved in soil water surrounding the rice root diffuses into the cell wall-water of the root ceils, gasifies in the root cortex and is transported in the gaseous state to the shoots via aerenchyma. In addition to the role of rice plant in CH4 emission, it also plays a significant role in CH_4 oxidationu because O_2 transported below the ground by plants, leaks out of the rhizosphere into the sediments, stimulating CH_4-oxidizing activity. Thus rice plants influence the methane dynamics in paddy soil by (1) providing substrate in the form of root exudates to methanogens and thus enhance the production of CH4; (2) transporting CH4 from soil to atmosphere (conduit effect), and (3) creating aerobic microhabitat in rhizosphere, which is suitable for growth and multiplication of methanotrophic bacteria.