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"Adapted from Houghton et al. (2001).

' Adapted from Raich and Scheslinger (1992). Turnover of soil C assumes 30% of soil respiration is derived from root respiration.

"Adapted from Houghton et al. (2001).

' Adapted from Raich and Scheslinger (1992). Turnover of soil C assumes 30% of soil respiration is derived from root respiration.

number of fractions with different turnover rates, rather than the one value given for total SOM. The overall turnover time reflects the rapid turnover of the recent plant residues. The majority of the SOM turns over much more slowly, with mean residence times often of thousands of years for the humic fractions protected by clay mineral associations (Paul et al., 2001).

role of methane in the c cycle

Methane comprises less than 1% of the global C budget. Methane is found as natural gas in fossil fuel deposits, as hydrates or clathrates compounds in ice (e.g., permafrost), in the deep ocean, and in the atmosphere. Soil microorganisms both produce (methanogens) and consume (methanotrophs) CH4. Microbial production of methane results from the decomposition of organic materials in the absence of oxygen. Carbon dioxide is used as an electron acceptor and a reduced organic compound is used as the donor. The reduction of CO2 will occur in soil under extended reduced conditions such as in flooded environments or where oxygen diffusion is severely limited such as within soil aggregates. Waterlogged soils such as rice paddies, wetlands, waste disposal sites, and ruminant stomachs are typical examples of methanogenic habitats (Table 12.5). The production and distribution of fossil fuels contribute significantly to CH4 emissions. Most of the annual flux of methane reacts with atmospheric hydroxyl radicals to form water and CO2. Soil organisms consume about 10 to 30 Tg CH4 year- \ far below that required to mitigate emissions from anthropogenic sources but of the same magnitude as the annual atmospheric

TABLE 12.5 Global Sources and Sinks for Methane

Tg CH4 year

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