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The term humic substances refers to an operationally defined, heterogeneous mixture of naturally occurring organic materials. These substances are ubiquitous in nature and arise from the decay of plant and animal residues in the environment. Humic substances are generally classified into humic acid, fulvic acid, and humin on the basis of their solubility in water as a function of pH. It has long been recognized that humic substances have many beneficial effects on soils and consequently on plant growth. Some of these effects have been well documented, but others, such as the alleged direct effect of humic substances on plant growth, are still a matter of controversy. The inherent complexity of humic substances and the inability of researchers to separate them into pure components imposes restrictions on the meaning of the term structure when applied to these materials; we must be satisfied with measuring the average characteristics of these substances. Fortunately, this is sufficient for understanding most of the properties of these materials and for explaining their effects in the soil. In addition to the inherent difficulty of working with a complex mixture, other factors that have contributed to the difficulty of studying these materials have been the inconsistent use of terminology and the previous lack of standard materials for comparison purposes. The recent establishment of standard humic and fulvic acids by the International Humic Substances Society remedies the latter problem.
Humic substances in soils differ significantly from those in stream waters or ground waters. Soil humic substances are of higher molecular weight, of greater 14C age, of greater percentage in aromatic C, of more intense color perC atom, and of higher polysaccharide content than their counterparts in streams and groundwaters. CPMAS 13C-NMR spectroscopy is the most useful and definitive characterization tool in demonstrating chemical differences among humic substances from different environments. Most so-called soil fulvic acids are really fulvic acid fractions, because traditional isolation methods do not remove specific compounds solubilized by the alkaline extraction. These unhumified components, such as carbohydrates and low-molecular-weight acids that can account for a large part of the soil fulvic acid fraction, can be removed by treatment with XAD-8 resins. For the samples included in this study, humic substances from all environments have in common (i) an extreme complexity of molecular structure; (ii) an abundance of acidic, ester, and phenolic functional groups; (iii) a predominance of aliphatic character; (iv) an ability to fluoresce; (v) a refractory nature to microbial decay; and (vi) an ability to form complexes with metal ions. Most soil humic substances are comprised primarily of humic acids, whereas fulvic acids constitute > 90% of stream humic substances. Humic acids differ significantly from fulvic acids in all these environments, and thus, the humic acid/fulvic acid separation appears to remain a valid characterization technique. Fulvic acids from all types of river waters appear to be remarkably similar, regardless of season, climatic conditions, or vegetation. Fulvic acids in soils vary slightly with soil type, method of extraction, and vegetative cover, but all have an abundance of variously degraded carbohydrate constituents. Groundwater fulvic acids have less intensive color, lower carbohydrate content, higher C content, lower N and O contents, and greater 14C ages than do stream and soil fulvic acids.
Soil humus is composed predominantly of two types of substances, humic substances and polysaccharides. Humic acids are the major extractable component of soil humic substances; fulvic acids are usually a small component of soil; and soil humin is a major nonextractable component. Humic acids are complex macromolecules consisting of an array of aromatic and aliphatic structures. Soil polysaccharides, nonhumic components, are discussed in this chapter because of their important role in soil structure and aggregation. They consist of a variety of sugar units of both plant and microbial origin. Decomposition of fresh organic residues is discussed in terms of the fate of the constituent carbon. Before residues can be considered a part of the true soil humus, they undergo profound transformation and no longer resemble the original material. An overview of some current hypotheses on the formation of humic acids and soil polysaccharides is presented along with possible mechanisms for their increased resistance to biodegradation. Use of model humic acid-type molecules have aided in elucidating some of the possible ways by which humic acids are synthesized and protected in the soil environment. Information on the characterization of humic acids and soil polysaccharides that has accumulated through the use of various degradative and nondegradative procedures is examined.
Four relatively new instrumental methods that are currently being used to study humic stances are (i) solution- and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy; (ii) electron spin resonance (ESR) spectroscopy; (iii) pyrolysis-mass spectrometry (Py-MS); and (iv) supercritical gas extraction (SCGE). 13C-NMR spectra provide us with an inventory of the different types of C (paraffinic, aliphatic and aromatic C, and C in CO2H, ketonic, and quinonoid groups) in soils and humic materials. ESR spectroscopy tells us about the symmetry and coordination of paramagnetic metals in metal-humic acid and mutal-fulvic acid complexes. Pyrolysis-soft ionization-mass spectrometry is a promising technique for identifying major humic components. Supercritical gas extraction is an important method for the relatively mild and efficient extraction of major humic components. These methods can be used separately or in combination and will, during the next decade, generate detailed and specific information on humic substances. This information will allow us to better understand and use these materials in agriculture. Examples illustrating the application of each of these methods are presented in this chapter.
An account is given of the nature and origin of nitrogen (N) in humic substances as a basis for understanding how N behaves in the soil-plant system. A significant amount of the soil N (> 50%) occurs as a structural component of humic substances, a portion of which is biologically stable and not readily available to plants. The N of humic substances may occur in the following types of structures: (i) as an amino acid attached to aromatic rings, (ii) as a bridge constituent linking quinone groups together, (iii) as part of a heterocyclic ring, (iv) as an open chain (−NH-, =N-) group, and (v) as peptides and proteins held through H-bonding. From 20 to 35% of the N applied to soils as fertilizers is retained in the soil in organic forms at the end of the growing season. This residual fertilizer N becomes increasingly unavailable for plant uptake during subsequent seasons, ultimately attaining equilibrium with the native humus N. The process whereby N becomes stabilized is discussed from the standpoint of the chemical nature of the stabilized N and the long-term N balance of the soil.
The sorption of nonionic organic compounds and pesticides on soil is here related to solute and solvent properties and to the following soil characteristics: organic matter content, mineral matter, and moisture content. A wide range of sorption behavior can be accounted for by considering the soil to be a dual sorbent, in which the mineral fraction of the soil functions as a conventional solid adsorbent and the organic matter functions as a partition medium. In aqueous systems, adsorption on mineral matter is suppressed by water, and the uptake by soil consists primarily of solute partitioning into the organic matter; this model agrees with the observed dependence of uptake by soil on soil organic matter content, the linearity of the isotherms, the small heat of soil sorption, and the absence of competition between solutes. By contrast, sorption from nonpolar organic solvents on dry soils is attributed to adsorption on soil minerals. Here the specific interaction of the adsorbate polar groups overcomes the weaker adsorptive competition by the (nonpolar) solvents, while the partition effect into soil organic matter is minimized by the relatively high solvency of the medium. Soil uptake from nonpolar solvents is depressed by soil moisture and approaches zero when the soil becomes fully saturated with water. The markedly higher sorption of organic vapors by dry and subsaturated soils, relative to that by wet soils, is ascribed to mineral adsorption, which predominates over the simultaneous uptake by partitioning into the organic matter, in which the mineral adsorptivity is influenced by the clay type and content. An increase of ambient humidity or soil moisture sharply depresses the vapor uptake because of adsorptive displacement by water on mineral surfaces. A small residual vapor uptake on water-saturated soil is attributed to the partitioning into the soil organic phase, which is similar to sorption by soil from aqueous systems. The effect of humidity on the activity and toxicity of nonionic organic pesticides in soil can be accounted for by the effect of humidity on sorption of the pesticide. The transport and fate of similar organic pollutants in the environment should likewise be influenced by ambient humidity.
Studies of the effects of humic substances on plant growth, under conditions of adequate mineral nutrition, consistently show positive effects on plant biomass. Stimulation of root growth is generally more apparent than stimulation of shoot growth. Both increases in root length and stimulation of the development of secondary roots have been observed for humic substances in nutrient solutions. The typical response curve shows increasing growth with increasing humic substance concentration in nutrient solutions, followed by a decrease in growth at very high concentrations. Shoots generally show similar trends in growth response to humic substances but the magnitude of the growth response is less. Foliar sprays can also enhance both root and shoot growth. The stimulatory effects of humic substances has been correlated with enhanced uptake of macronutrients. Humic substances can complex transition metal cations, which can sometimes result in enhanced uptake and sometimes result in competition with the roots resulting in decreased uptake. A small fraction of lower molecular weight components in humic substances can be taken up by plants. These components seem to increase cell membrane permeability and may have hormone-like activity. In soils, addition of composts can stimulate growth beyond that provided by mineral nutrients presumably because of the effects of humic substances. Addition of Fe-enriched organic materials can alleviate high-lime chlorosis. Soil additions of prepared humic substances is not economical, but the response to foliar sprays has the potential to be economical because of the relatively small quantities needed.
Carbon-13 nuclear magnetic resonance (NMR) spectroscopy and pyrolysis field ionization mass spectrometry have been used to evaluate the differences in organic matter composition of a very fertile, man-made Amazon soil (Terra Preta Do Indio) and a relatively barren Oxisol derived from the same parent material. The most pronounced difference found is the high aromaticity of the Terra Preta humus which is considered to be due to mineralization and humification of large amounts of nutrientrich organic materials. Carbon-13 NMR spectroscopy also has been applied to the examination of changes in organic matter composition due to clearing of forests and cultivation. Five neighboring soils in Yucatan, Mexico, differing only in land use have been investigated. The results indicate that intensive cultivation leads to a strong increase in the aromaticity of the organic matter of these soils.
Humic and fulvic acid fractions obtained from sewage sludges show several distinguishing chemical and structural characteristics when compared to soil-derived humics including: (i) higher N contents, (ii) lower C/N ratios, (iii) higher H/C ratios indicating a higher fraction of aliphatic components, and (iv) lower carboxyl group acidity. Infrared spectra of sludge humic and fulvic acids show the presence of associated protein and aliphatic materials like fats and waxes. Several nonhumic components, accounting for 30 to 55% by weight, have also been identified in sludge fulvic acid fractions. These include: (i) amino acids, (ii) hexosamines, (iii) neutral sugars, and (iv) anionic surfactants. The presence of sodium lauryl sulfate and other organic ester sulfate detergents (anionic surfactants) results in a much higher S content in sludge fulvic acid fractions as compared to soil fulvic acid fractions. Humic and fulvic acids from sewage sludges have complex metal binding sites involving oxygen-containing chelating groups and mixed nitrogen/oxygen ligand systems. Humic and fulvic acid fractions obtained from sludge-amended soils show characteristics of sludge humic and fulvic acid fractions. These include (i) higher S contents; (ii) presence of lauryl sulfate type surfactants; and (iii) increased N contents, higher H/C ratios, and lower acidity. The association of sludge-derived protein and aliphatic materials with humic materials from sludge-amended soils can be seen clearly in their infrared spectra. With time after sludge application, the effects of sludge amendment on soil humics become less apparent.
Basic principles of solid state nuclear magnetic resonance (NMR) spectroscopy relevant to the study of the structure of organic matter of whole soils or solid fractions of soils are outlined. These include cross polarization, magic angle spinning, relaxation phenomena, and decoupling. There are problems in obtaining quantitative data by cross polarization techniques since there is more than one spin lattice relaxation time in the rotating frame. Selective relaxation can be used, however, to identify specific functional groups in soil. Spectra of wood, cellulose, lignin, and their decomposition products can be related to spectra obtained from soil organic matter. The most notable feature about 13C-NMR spectra of whole soils is their variability, and it is shown that this is due to soil-forming factors such as climate and vegetation. The isotopes 31P and 15N in soils can also be studied by NMR spectroscopy and can be used to trace the incorporation of these elements into soils.
Nuclear magnetic resonance spectroscopy (NMR) is a nondestructive technique which can provide information about the types of forms of C, P, N, and other elements in materials and hence has great potential in soil science. The application of some aspects of NMR spectroscopy to the analysis of soil organic matter is the subject of this chapter. A full description of the NMR technique is beyond the scope of this chapter and the reader is referred elsewhere for a more detailed treatment (Becker, 1980; Fukushima & Roeder, 1981; Fyfe, 1983; Shaw, 1984), in particular to the text specifically written for geochemists (Wilson, 1987). However, a fairly lengthy description is necessary here in order that the reader who has little NMR background can follow the text. Thus, the chapter has a secondary function: that is to act as an NMR spectroscopy primer for the practicing scientist. A list of symbols and definitions is shown in the Appendix.
The roles of humic substances in soil and crop sciences, discussed in detail in the preceding chapters of this book, are summarized here. The topics addressed include: the extraction, composition, characterization, and formation of soil humic substances. Particular attention is focused on the application of 13C-NMR spectroscopy to these materials. Other subjects addressed are the stabilization of soil organic matter through humification, sorption of nonionic compounds by soil organic matter, and sewage sludge humic materials. The influences of humic substances on soil fertility and plant growth are also discussed. The chapter concludes with an ecological view of humic substances that provides a rationale for their highly intractable nature.