There are literally thousands of chemical and biochemical compounds involved in catabolism. Viewed in an ecological context, however, they can be classified into two functional categories: (1) Primary compounds, those which are directly derived from plant, microbial, or animal tissues, and (2) secondary compounds, those which are produced as a result of organic matter-mineral interactions, usually resulting in small or large chemical changes in chemical bonds or degree of aromaticity.
Both categories comprise a few major types (or groups) of compounds: soluble, or labile, versus relatively insoluble (in water) nonlabile, or resistant, compounds. Compounds in the former category include organic acids, amino acids, and simple sugars. Compounds in the latter category include lignin, cellulose, cutins, and waxes. One should also consider biochemical versus biological bond types as defined by McGill and Cole (1981). These reflect the differences between ester linkages, designated R-C-O-O-R, which yield energy when broken, and the car-bonyl C-N, C-P, or C-S bonds, which require energy to be cleaved, yielding nutrients to the microbes (Newman and Tate, 1980).
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