Biomoleculesare compounds synthesized to sustain the life cycles of organisms. In soil humus, they are usually products of litter degradation, root excretion, and microbial metabolism, ranging in molecular structure from simple organic acids to complex biopolymers. Organic acids are among the best-characterized biomolecules. Table 3.1 lists five aliphatic (meaning the C atoms are arranged in open-chain structures) organic acids associated commonly with the soil microbiome. These acids contain the unit R—COOH, where COOH is the carboxyl groupand R represents either H or an organic moiety. The carboxyl group can lose its proton easily within the normal range of soil pH (see the third column of Table 3.1) and so is an example of a Brønsted acid. The released proton, in turn, can attack soil minerals to induce their decomposition (see Eq. 1.2), whereas the carboxylate anion (COO-) can form soluble complexes with metal cations, such as Al3+, that are released by mineral weathering [for example, in Eq. 1.7, rewrite oxalate, C2O42-, as (COO-) 2]. The total concentration of organic acids in the soil solution ranges up to 5 mM. These acids tend to have very short lifetimes because of biocycling, but they abide as a component of soil humus, especially its water-soluble fraction, because they are produced continually by microorganisms and plant roots. Formic acid (methanoic acid), the first entry in Table 3.1, is a monocarboxylic acid produced by bacteria and found in the root exudates of maize. Acetic acid (ethanoic acid) also is produced microbially—especially under anaerobic conditions—and is found in root exudates of grasses and herbs. Formic and acetic acid concentrations in the soil solution range from 2 to 5 mM. Oxalic acid (ethanedioic acid), which is ubiquitous in soils, and tartaric acid (D- 2,3-dihydroxybutanedioic acid) are dicarboxylic acids produced by fungi and excreted by plant roots; their soil solution concentrations range from 0.05 to 1 mM. The tricarboxylic citric acid (2-hydroxypropane- 1,2,3-tricarboxylic acid) is also produced by fungi and excreted by plant roots. Its soil solution concentration is less than 0.05 mM.
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