RELATIONSHIP OF MICROBIAL PROCESSES 311 



1972), they have the potential for formation of soluble complexes with metals, 

 particularly in dilute solutions. Small quantities of metal fulvates, thought to be of lower 

 molecular weight than the humates. may be present in soil solution. A nondialyzable 

 material with infrared absorption spectra and elemental analyses similar to fulvic acids 

 was isolated from a dilute salt (O.OIM KBr) extract of a mineral soil by Geering and 

 Hodgson (1969). The material exhibited a concentration equivalent to 2.5% of a 

 dialyzable fraction but was more effective in complexing copper and zinc. 



Nonhumic Substances with Potential for Metal Complexation. Lower molecular-weight 

 biochemicals of recent origin have been implicated in metal complexation and 

 solubilization in soil. These materials represent (1) components of living cells of 

 microorganisms and plant roots and their exudates and (2) the entire spectrum of 

 degradation products which uUimately serve as the building units of the soil humic 

 fraction. The quantity and composition of these materials will vary with soil, vegetation, 

 and environmental conditions (Alexander, 1961; 1971). Readily decomposable wastes 

 disposed to soil under conditions appropriate for microbial growth may, for example, 

 result in immediate and marked increases in organic materials identified in (1) and longer 

 term increases of materials in (2). Conversely, toxic materials may have the opposite 

 effects. The specific compounds produced will be dependent on the properties of the 

 waste and soil environmental conditions after disposal (Routson and Wildung, 1969). 



Althougli the concentration of the transuranic elements and other metals soluble in 

 the soil solution or in mild extractants is low, often near-minimum detectable levels, the 

 major portions of copper and zinc were shown to be associated with low-molecular- 

 weight components. Most of the titratable acidity of this fraction was attributed (Geering 

 and Hodgson, 1969) to aliphatic acids (<pH 7.0) and amino acids (>pH 7.0). 



The production, distribution, and action of organic acids in soil were reviewed by 

 Stevenson (1967). A wide range of organic acids is produced by microorganisms known 

 to be present in soil. These include (1) simple acids, such as acetic, propionic, and 

 butyric, which are produced in largest quantities by bacteria under anaerobic conditions; 

 (2) carboxyHc acids derived from monosaccharides, such as gluconic, glucuronic, and 

 a-ketogluconic acids,, which are produced by both bacteria and fungi; (3) products of the 

 citric acid cycle, such as succinic, fumaric, maUc, and citric acid, which are common 

 metabolic excretory products of fungi; and (4) aromatic acids, such as p-hydroxybenzoic, 

 vanillic, and syringic acids, wliich are thought to be fungal decomposition products of 

 plant lignins. A variety of organic acids have also been reported in root exudates. 



Amino acids are the other important group of compounds identified in significant 

 quantities in the soil solution by Geering and Hodgson (1969) which may be expected to 

 exhibit strong affinity for metals. The qualitative and quantitative aspects of amino acids 

 and other nitrogenous components in soils have been reviewed by Bremner (1967). It was 

 concluded that soil acid hydrolysates do not differ greatly in amino acid composition, but 

 quantitative differences may occur with differences in soil, cHmatic, and cuhural 

 practices. A number of acidic and basic amino acids have been reported in soil. However, 

 it appears that the major portion of amino acid-N that is present in hydrolysates is in 

 (1) the neutral amino acids, glycine, alanine, serine, threonine, valine, leucine, isoleucine, 

 and proline; (2) the acidic amino acids, aspartic acid and glutamic acid; and (3) the basic 

 amino acids, lysine and arginine. Most of the amino acids detected in soil hydrolysates 

 have also been shown to exist free in small quantities in soils with levels seldom exceeding 

 2/ig/g. In the soil solution (Geering and Hodgson, 1969) neutral amino acids also 



