Nonhumic substances

Nonhnmic substances are those recognizable as known biochemical compounds, such as amino acids, carbohydrates, fats, organic acids, resins, and waxes these are usually under degradation conditions and are thus relatively short lived (Senesi and Loffredo 1998 Baldock and Nelson 1999). The main categories of nonhumic materials are the following biopolymers  

Polypeptides and other N-containing compounds N is a major nutrient and is unique in that organic N accounts for more than 90% of the total N in topsoil. A variety of nitrogen-containing compounds can be found in soils, including amino acids and amino acid-derived substances (constituting about 

FIGURE 7.6 The structure of cellulose, a typical polysaccharide, which is a structural constituent of most plants it is composed of D-glucopyranose units. 

FIG U RE 7.7 Some examples of nitrogen-containing compounds found in soils (a) a segment of a polypeptide chain, (b) A structural unit of chitin, an amino sugar polymer found in plant structures and in insect exoskeleton. 

Structure of polyester chains and some monomers typically found in cutin and suberin, the main components of plant cuticle. 

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Nonhumic substances Compounds belonging to known classes of biochemistry, such as amino acids, carbohydrates, fats, waxes, resins, and organic acids. Humus probably contains most, if not all, of the biochemical compounds synthesized by living organisms... 

Bano et al. (1998) Nonhumic substances (wetland) Artificial (solar simulator) Bacterial biomass + 143%... 

Obernosterer and Herndl Nonhumic substances Natural Bacterial cell number 50% and 0%... 

Soil and related environments are both an important natural habitat of biota and a natural reservoir of biotic debris consisting of plant remains and dead animals and microorganisms. With time, dead remains are subject to continuous turnover, either mineralized or transformed to diverse organic components which are termed humus. This process is referred to as humification. Humus is composed of humic substances plus nonhumic substances that have become stabilized and are thus an integral part of soil and related environments (Table 2.1). 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Figure 10.5. Comparison of the mass balance (in % DOC) for different fractions obtained by the XAD-8 procedure (5a) (FA, fulvic acid HA, humic acid NHS, nonhumic substances) and by ultrafiltration (5b) (K, concentrate P, permeate cutoff, 4000g/mol). Abbreviations used HO, Lake Hohloh, brown water ABV, wastewater effluent numbers, sampling occasion.

Nonspecific organic matter or nonhumic substances These are produced during the decomposition of plants and animals, and their structure is known and well defined. Carbohydrates, organic N- and P-compounds, lignine, and organic acids are mentioned here. 

Humic substances. In most soUs, humic substances comprise —60-80% of SOM. Humic substances are distinguished from nonhumic substances in that they are unique to the soil or sediment environment and are composed of relatively high-molecular-weight compounds that can neither be characterized as biopolymers of microorganisms nor higher plants (Stevenson, 1994). 

The humic acid/fulvic acid ratios of the surface layers from several great soil groups are shown in Table 3. In agreement with the above, soils representative of the Mollisols (Chernozem and Chestnut) have the highest ratios. One difficulty in interpreting published data on humic acid/fulvic acid ratios is that seldom has allowance been made for nonhumic substances present as impurities, particularly in the fulvic acid fraction. 

Evidence that most of the soil organic matter occurs in association with clay has come from studies where unbound organic matter, consisting of free humic and nonhumic substances plus undecayed or partially modified plant remains, is removed by flotation in a liquid of density intermediate between the free material and the clay-organic complex (see Greenland, 1965b). Solutions of density between 1.8 and 2.0 have been used, such as a benzene-bromoform mixture. Elutriation and sieving methods have also been applied. 

Biochemical compounds such as carbohydrates, proteins (amino acids), and lipids present in humic substance fractions (humic acid, fulvic acid, and humin) pose a problem in determining and characterizing humic substances in various environments, especially in freshly deposited lake sediments where relatively large amounts of those biochemicals are present. Common separation methods cannot separate true humic substances from non-humic substances. According to Riffaldi and Schnitzer (1972b), 6N HCl hydrolysis efficiently removes nonhumic substances from soil humic acid. However, the following questions remain unresolved ... 

Following the extraction of humic substances from soil media it is necessary to purify the humic substances by separating them from the nonhumic substances. Used in this sense purification is the removal of materials such as carbohydrates, proteins, lipids, low-molecular-weight compounds, and so on which have been co-extracted with the humic substances. This whole process is called isolation and is dealt with in Chapter 13 by Hayes. 

Organic solvents have long been used for extraction and sequential extraction, which is fractionation of a sort (Flaig et al., 1975 Schnitzer, 1978). While the direct use of organic solvents in fractionation has not been widespread, nonetheless, the technique has received some attention. For instance, the separation of hymatomelanic acid from precipitated humic acid is obtained by extraction with ethanol (Oden, 1919). Ethanol has been used to bring about fractional precipitation by addition to alkaline solutions of humic acid (Kyuma, 1964 Kumada and Kawamura, 1968). There is no reason why other water-miscible solvents such as acetone and methanol should not be used in this way. Solvents that are highly immiscible with water (e.g., hexane and benzene) do not appear to remove any substantial fraction of humic substances. These are perhaps best used to remove nonhumic substances (such as fats and waxes) prior to extraction. However, recent work by Allen and MacCarthy (personal communication) has shown that more polar water-immiscible solvents, such as methyl isobutyl ketone and diethyl ether, can be used successfully to purify and fractionate humic substances. 

I There is no clear distinction between humic and nonhumic substances in... 

H/C ratios are clustered around 1.0 for most soil and aquatic humates and fulvates. Lake and marine sedimentary humic substances have somewhat higher H/C ratios than their soil or water counterparts (Ishiwatari, 1975a). A plot of EJE(, ratios versus H/C ratios shows a direct correlation for terrestrial humic acids (Ertel and Hedges, 1983). The magnitude of the EjEe ratio is inversely proportional to the degree of condensation or the molecular weight (Chen et al., 1977). Ratios above 1.3 indicate that the material may be a nonhumic substance. 

Similarly, polyphenols are synthesized by microorganisms from nonhumic substances, which are subsequently converted to quinines through enzymatic reactions. The conversion of polyphenols formed from both sources to quinines is catalyzed by oxygen-requiring phenol oxidase enzymes. Quinones polymerize with or without amino compounds to from complex humic substances (Figure 5.15b). 

SOM is classified into two main categories humic and nonhumic substances (MacCarthy, Clapp, and Malcolm 1990 Senesi and Loffredo 1998 Baldock and Nelson 1999 Sparks 2002 Brady and Weil 2007). 

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