ABSTRACT
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.
