We will now consider system-level catabolism, or dissipation and transformation of energy (the roots of catabolism, cata bolos, mean "breaking-down activity"). The transfer of energy from the primary producers into organisms farther along the food chain supports a wide range of heterotrophs. The production of new body tissues by het-erotrophs from primary production is called secondary production. If the plant food sources are living, the linkages are called a grazing food chain. Conversely, if the contributions from net primary production (NPP) are dead, the sequence is termed a detrital food chain.
This difference in food chains has some impact on system function, in that grazing food chains have a direct feedback, whereas detrital food webs have only indirect effects. Soil food chains and webs are discussed further in Chapter 6.
The array of energy dissipators, or heterotrophs, in soil is incredibly diverse. The size range goes from less than 1 micrometer (|im) in length (bacteria) to the largest fossorial mammals, such as aardvarks or badgers, and giant earthworms that reach 2 meters (m) in length (Lee, 1985). Larger entities include ant and termite colonies, considered by some to be a "superorganism" (Emerson, 1956). Larger yet are supercolonies of one organism of uniform genetic material such as the extended mycelium of a fungus in Michigan that extended over more than 7 hectares (Smith et al., 1992).
All heterotrophs, of whatever size or volume, are involved in ingesting organic carbon and associated nutrients and assimilating them into carbohydrates, lipids, and proteins. Using a portion for production of new body tissue, an extensive amount (40% or more) of the chemical bond energy is lost as metabolic heat and evolved carbon dioxide (CO2). Assimilated NPP (e.g., plant carbohydrate) is catabolized according to the general formula:
The more general formulation thus becomes:
Stoichiometrically, this is the reverse of photosynthesis, which was discussed in Chapter 2.
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