Introduction

Sources of atmospheric CH4 in the biosphere have until recently been attributed to originate from strictly anaerobic microbial processes in wetland soils and rice paddies (Chapters 3 and 8), the guts of termites and ruminants (Chapters 5 and 9), human and agricultural waste (Chapter 10), and from biomass burning (Chapter 7), fossil fuel mining (Chapter 12) and geologic sources including mud volcanoes and seeps (Chapter 4). However, in early 2006, Keppler et al published a surprising report of direct CH4 emission from vegetation foliage under aerobic conditions. Their study enclosed samples of detached leaves, air-dried leaves, intact plants and the plant structural component pectin in CH4-free air inside closed vials or chambers and measured the build-up of CH4 in the enclosure. This revealed rates of CH4 emission from a range of air-dried tree and grass leaves from C3 and C4 plants in the range 0.2-3 nanograms (ng) per gram dry weight per hour (g-1 d.wt. h-1) at 30°C but increasing to much higher rates of 12-370ng g-1 d.wt. h-1 for intact plants. Methane emission rates increased by a factor of three to five when chambers were exposed to natural sunlight. The emissions of air-dried leaves appeared to be non-enzymatic as they increased over the range 30-70°C (Figure 6.1) and above the threshold of 50-60°C (Berry and Raison, 1982) at which plant enzymes are denatured. Furthermore, air-dried leaves that were sterilized by prior exposure to gamma radiation emitted the same amount of CH4 as the untreated leaves suggesting that microbial production was not involved.

Although these rates of emission were small, Keppler et al (2006) also completed a rough extrapolation to estimate a total annual global emission of CH4 from this living vegetation source by using mean sunlit and dark emission rates for leaf biomass scaled by day length, duration of growing season and total net primary productivity in each biome. Their estimate of 62-236Tg (1Tg = 1012g) CH4 yr-1, with the largest contribution of 46-169Tg CH4 yr-1 from tropical forests and grassland, was observed to equate to 10-40 per cent of the known annual CH4 source strength (IPCC, 2007). Plant litter was estimated to contribute 0.5-6.6Tg CH4 yr-1. Consequently, these first observations of

Keppler et al (2006) caused intense interest, considerable debate and some scepticism among the scientific community and the media (Schiermeier, 2006a, 2006b) leading to further experimental studies and a wider consideration of their implications for the global CH4 budget and greenhouse gas mitigation options (Lowe, 2006; NIEPS, 2006).

Figure 6.1 (a) Release of CH4 from air-dried leaves of ash (Fraxinus excelsior) and beech (Fagussylvatica) in the temperature range 30-70°C under aerobic conditions; (b) Closed leaf chambers used for whole plant measurement of CH4 emissions

Source: (a) adapted from Keppler et al (2006)

Figure 6.1 (a) Release of CH4 from air-dried leaves of ash (Fraxinus excelsior) and beech (Fagussylvatica) in the temperature range 30-70°C under aerobic conditions; (b) Closed leaf chambers used for whole plant measurement of CH4 emissions

Source: (a) adapted from Keppler et al (2006)

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

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