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allows them to gain energy by a wider range of mechanisms than exist in higher organisms. Third, mechanisms operating at long (geologic in some cases) time scales sequester substrates in locations isolated from active microorganisms.

Soils are nature's integrators; they form on the surface of the earth where the cycles of matter and the transfer of energy all meet. Soils interact reciprocally with the biosphere, hydrosphere, lithosphere and atmosphere (Fig. 9.1).

Only in soils do all four fundamental spheres interact. Such interactions entail biological, chemical, biochemical, and physical transformations and biological and physical translocations. Soil organisms and especially soil microorganisms are intimately involved in biological and biochemical transformations. They are both sinks for elements and catalysts to speed transformations of elements. Hence physiology and biochemistry of soil organisms is fundamentally important to understanding earth systems. Such a myriad of interactions may appear chaotic, random, or complex beyond understanding. Not so. Nature may be complex but systematic, and ways exist to organize knowledge. The requirement to conserve mass means that earth systems must operate in cyclic ways, so cycles become helpful organizing principles. Additionally size scales and time scales are related. A way to look at this relationship is through the lens of cycles embedded within cycles. The shorter the cycle, and the smaller the mass of material within it, the faster it must operate to connect to a larger cycle with a greater mass of material moving through it, but at a slower pace. Three groups of cycles are useful: geochemical cycles, biogeochem-ical cycles, and biochemical cycles. As used here, geochemical cycles are analogous to macroscale systems, are dominated by exchange of materials among ecosystems, and are represented by processes occurring in zonal soils, and on a global scale they represent cycles of volcanism, weathering, and erosion. Biogeochemical

FIGURE 9. 1 Soil, or the pedosphere, as the integration of four fundamental spheres. (Reproduced from McGill (1996), with permission from SBCS and SLCS.)

cycles relate to mesoscales, typically exchange materials within ecosystems, are dominant in soil landscapes, and are represented by element cycles such as the N cycle. Biochemical cycles operate within individuals or single cells, come close to the microscale, and dominate within soil profiles or aggregates and can be represented by the tricarboxylic acid cycle (TCA cycle).

metabolic classifications of soil organisms

How can organisms, their physiology, and their functions be organized? Criteria with which to classify soil microorganisms include size, morphology, molecular genetics or physiology, and metabolism. A metabolic classification based on source of energy, electrons, and C has practical value (Tables 9.1 and 9.2). Phototrophs

TABLE 9.1 Metabolic Classification of Soil Microorganisms

Metabolic group

Electron source Electron acceptor"

C source

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