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matter

Primary and secondary mineral compartment

Weathering

Weathering

Precipitation

Precipitation

Elemental soil cycle

Available nutrient compartment

Exchange sites

Soil solution

Meterologic

Geologic

Biologic

Meterologic

Geologic

Biologic

FIGURE 15.1 A conceptual representation of a generic elemental biogeochemistry cycle (with permission from Likens and Bormann, 1999).

Biologic fluxes result when chemicals or energy gathered by organisms in one ecosystem are deposited in another (e.g., entrainment of litter material from above the soil surface to subsurface layers). The nutrient or metal element may occur in four compartments within the soil ecosystem: (1) atmosphere, (2) living and dead organic matter, (3) available nutrients, and (4) primary and secondary minerals. Microbially mediated reactions that transform these elements include: (1) mineralization and immobilization, reactions that transform the element from organic to inorganic and inorganic to organic forms, respectively; (2) reduction and oxidation, reactions that involve the transfer of electrons; (3) solubilization, reactions by which relatively insoluble materials are rendered soluble and therefore available to plants or microorganisms; (4) volatilization, reactions that transform an element to a volatile or gaseous form; and (5) detoxification, reactions that reduce the toxicity of an element to the microorganism in question. The last group of reactions includes redox reactions as well as alkylation reactions.

The ability of soil microorganisms to oxidize or reduce several elements has likely developed in response to changing environments over the course of evolution and is now evident in gradient environments such as soil where O2 is more or less available due to water- versus air-filled porosity (Fig. 15.2). In aerobic environments, stoichiometry may integrate other element cycles, but energy generation is dominated by a union between the C and the O cycles. However, when the redox potential (Eh) decreases, a number of other element cycles become more closely integrated as alternative electron acceptors (e.g., Fe3+, Mn4+, NOp, SO4~) are utilized by various groups of organisms in microbially mediated oxidation and reduction reactions. An example of element cycle integration is the autotrophic facultative anaerobic bacterium Thiobacillus denitrificans, which is capable of oxidizing sulfide to elemental S using nitrate as its electron acceptor and carbon dioxide as its sole C source under anoxic conditions. The organism can accumulate S extra-cellularly and converts the nitrate to nitrogen gas. Other examples of element cycle integrations are illustrated under Environmental Significance of P, S, and Metal Biogeochemistry, in which several examples of the unification of Fe and S cycles during the processes of acid mine drainage formation and corrosion are given. Our improved understanding of the role of microorganisms in the biogeo-chemistry of many elements, through the application of modern molecular and genetic tools, will certainly continue to contribute to the development of strategies to protect the environment from contamination and to extract and recycle valuable resources in a sustainable manner.

phosphorus

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