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Decomposition is the process by which organic matter and its associated nutrients and minerals are recycled through the biosphere by soil organisms. Three-fourths of all terrestrial carbon is present in soil, and various soils have different levels of carbon storage and decomposition rates.
Put simply, decomposition is the breakdown of complex organic molecules into simpler organic molecules by soil organisms, particularly bacteria and fungi. Decomposition begins when dead organic material such as plant leaves, roots, and wood, soil animals, soil microbes, plant root exudates, and soil animal excretions are deposited onto and into soil. Plant material accounts for the majority of organic matter, and thus carbon, added to soil organic matter pools in most ecosystems. Soil animals such as worms, insects, and small mammals mix the surface litter with deeper soil layers. This physical mixing breaks the fresh organic matter into smaller pieces in a process termed comminution, which increases the surface area available for microbial attack. Decomposer bacteria and fungi then become more active on the organic matter fragments, and decomposition proceeds more rapidly. The bacteria and fungi release specialized enzymes that cleave (depolymerize) bonds between complex organic molecules. Depolymerization breaks complex carbon compounds into simpler molecules such as single glucose units that are more easily metabolized by the microbes as energy and carbon sources. Some common decomposer bacteria include Bacillus, Pseudomonas, and Clostridium, which are active in decomposing cellulose. The brown and white rot fungi (Basidiomycetes) are common fungal decomposers that specialize in decomposing celluloses and lignin, respectively. Microbes also decompose organic molecules in soil that are not derived directly from plant matter, such as organic compounds exuded from soil animals, dead bacteria, and fungal hyphae, or carbon stored as humus (described below). Soil animals such as earthworms that ingest organic matter aid in decomposition by inoculating organic matter with decomposer microbes, and by dispersing microbial inocula to organic matter hotspots.
The simpler organic compounds that result from decomposition may be used by microbes for energy or growth, mineralized by other microbes into inorganic nutrients and minerals, or incorporated into highly complex organic carbon compounds called humus that decompose very slowly. The byproduct of microbial decomposition under aerobic (sufficient oxygen) conditions is carbon dioxide (CO2), which may remain in soil pores, dissolve in soil water to form carbonic acid, or diffuse from soil into theatmosphere. Thus, the rate of gaseous CO2 released from soil is often used as a proxy to estimate microbial decomposition rates.
Decomposition rates vary seasonally and across scales from landscapes to biomes. The rate of organic matter decomposition depends upon organic matter quality (which influences how easily it is decomposed); community composition and activity of decomposer microbes and other soil organisms; and soil physical conditions, particularly temperature and moisture. Decomposition generally proceeds faster in warm, moist climates with deciduous vegetation, and slower in cooler, drier climates with evergreen vegetation. Recent scientific studies suggest that warming of the global climate may speed up the decomposition of organic matter stored in soils of high-elevation and high-latitude ecosystems such as boreal forests and tundra.
- Dirk Bryant, Daniel Nielsen, and Laura Tangley, The Last Frontier Forests: Ecosystems and Economies on the Edge (World Resources Institute, 1997) Clare Foster, Compost (Cassell, 2002);
- Killham, Soil Ecology (Cambridge University Press, 1994).