The sequence of biological organic matter oxidation (e.g., Schlesinger, 1997) takes place in the following order: by oxygen, by nitrate, by manganese dioxide, by iron (III), by sulphate, and by carbon dioxide. This means that oxygen, if present, will always out compete nitrate which will out compete manganese dioxide, and so on. The amount of exergy stored as a result of an oxidation process is measured by the available kJ/mole of electrons which determines the number of adenosine triphosphate molecules (ATPs) formed. ATP represents an exergy storage of 42kJ/mole. Usable energy as exergy in ATPs decreases in the same sequence as indicated above. This is as expected if the exergy storage hypothesis were valid (Table 6.4). If more oxidizing agents are offered to a system, the one giving the highest storage of free energy will be selected.
In Table 6.3, the first (aerobic) reaction will always out compete the others because it gives the highest yield of stored exergy. The last (anaerobic) reaction produces methane; this is a less complete oxidation than the first because methane has a greater exergy content than water.
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