Ssj50107 986..994

The classical methods for the isolation of soil organic matter (SOM) components use aqueous base or neutral salt solutions, and combinations of aqueous base and pyrophosphate. Organic solvents have been rarely used, largely because of difficulties in recovering solutes. This review provides the relevant chemistry of aqueous and organic chemicals relevant to extracting and fractionating the components of SOM that are bound and are not bound by the soil mineral surfaces. Uses of aqueousmedia to separate the SOMcomponents on the basis of charge density differences are described. Combinations of aqueous base and urea enhance the isolations of the SOM components that have a high degree of polarity [humic and fulvic acids (HAs and FAs), polysaccharides, peptides]. However, the nature of the associations between the solute molecules has prevented the isolation of any purified SOM component. Properties are listed of organic solvents that have potential for the isolation of SOM components, and novel procedures are described for the recovery of the SOMcomponents dissolved in organic solvents. The procedures include the uses of resinswith varying degrees of polarity and the recovery of the SOMcomponents in aqueousmedia. No satisfactory solvent system has been found that can isolate all of the huminmaterials sorbed by the inorganic colloids.However, procedures are outlined that can extract much of thematerial classified as humin in the classical definitions. Because the polar components of the SOM can now be removed, it is likely that the compositions of the nonpolar humins strongly held by the mineral colloids will be resolved using procedures such as pyrolysis–mass spectrometry. STEVENSON (1994) considered SOM to consist of all of the organic materials in soils. Hayes and Swift (1978) regarded the complete soil organic fraction to be made up of live organisms and their partly decomposed and completely transformed remains, as well as those of plants, but they considered SOM as a more specific term for the heterogeneous mixture of nonliving components resulting from the biological and chemical transformations of organic debris. The transformations are known as humification processes, and the final products as humus. This is similar to Stevenson’s (1994) definition that regarded humus as the total of organic compounds in soil which have some resistance to biodegradation but are exclusive of decaying plant and animal tissues, their partial decomposition products, and the soil biomass. Soil organic matter or humus is perhaps the most complex material in nature. Most emphasis has focused on the humic substances (HSs). In the classical definitions, HSs have been regarded as a series of relatively high-molecular-weight dark-colored substances formed by secondary synthesis reactions (Stevenson, 1994), and as a “category of naturally occurring, biogenic, heterogeneous organic substances that can generally be characterized as being yellow to black in color, of high molecular weight, and refractory” (Aiken et al., 1985). In classical definitions, unaltered biological molecular components of plants and animals, though components of humus, would not be considered to be HSs. Thus, unaltered flavonoids, tannins, terpenes, sporopollenins, and large aliphatic molecules such as algaenans, cutans, suberans (Derenne and Largeau, 2001), though components of humus, would not be regarded as HSs. There is increasing emphasis on the presence of charcoal, or char, from the burning of vegetation (Skjemstad et al., 1996; Swift, 2001). Although some charcoal compounds fall within the operational definitions of HSs, based on solubilities data, these cannot be considered to be true HSs. The operational definitions of SOM fractions based on solubilities were first introduced by Sprengel (1837). Soil scientists define HAs as humus materials that are soluble in aqueous alkaline solutions but precipitate when the pH is adjusted to 1 (water scientists consider the precipitates at pH 2 to be HAs). There are, of course, many nonhumic components (e.g., some proteins) that are precipitated under similar conditions, and so the precipitates at pH 1 might be considered as the HA fraction. Fulvic acids remain in solution after the aqueous alkaline extracts are acidified. Thesematerials are best referred to as the FA fraction. The FA standards of the International Humic Substances Society (IHSS) were processed by passing the acidified solutions on to XAD-8 resin [(poly)methylmethacrylate], which retain the FAs and allow polar components to pass through the resin. The FAs are recovered in 0.1MNaOH, andH–exchanged by passing through IR-120 (styrenedivinylbenzene with sulfonic acid functionality, H–exchanged) resin, followed by freeze drying. Polar, nonhumic components, such as various peptides, saccharides, and some small organic chemicals, are not retained on XAD-8 but may be held by XAD-4 (styrenedivinylbenzene) resin. To some extent, polar contaminants may also be removed fromHAs using XAD-8 resin technology. Häusler and Hayes (1996) removed some of the saccharide and peptide components from HAs in a Sapric Histosol using acidified dimethylsulfoxide [DMSO1 12% (v/v) of 12M HCl]. The acidified solution was passed on to a XAD8 resin which retained the HAs. The DMSO was washed through with acidified water, then with distilled water, and the HAs were recovered in 0.1MNaOH. These were then H–exchanged by passing through IR-120, and then freeze dried. Chemical and Environmental Sciences, Univ. of Limerick, Ireland. Received 4Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:986–994 (2006). Symposium: Meaningful Pools in Determining Soil C and N Dynamics doi:10.2136/sssaj2005.0107 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DMF, N,N-dimethylformamide; DMSO, dimethylsulfoxide; DOSY, diffusion ordered spectroscopy; EDA, ethylenediamine; EF, electrostatic factor; HA, humic acid; HS, humic substance; IHSS, International Humic Substances Society; NMR, nuclear magnetic resonance; SOM, soil organic matter. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 986 Published online April 19, 2006