SREL Reprint #2028

Characterization of dissolved and colloidal organic matter in soil solution: A review

Bruce E. Herbert1 and Paul M. Bertsch2

1Texas A&M University, College Station, Texas
2University of Georgia, Savannah River Ecology Laboratory, Aiken, South Carolina

Summary: Soil solution dissolved organic matter (DOM) is intimately tied to many biogeochemical processes in soils such as mineral weathering, translocation of oxides, humus, and silicate clays, and nutrient cycling. In addition, many processes controlling the fate and transport of organic contaminants in soils, such as sorption, chemical degradation, bioavailability, and transport, are mediated by the presence of DOM in soil solution. Because of the difficulty in collecting soil solution DOM, water soluble organic matter (WSOM) has been used as a surrogate for soil solution DOM. While there are probably some differences in their respective properties, WSOM is a better surrogate for soil solution DOM than base-extracted fulvic or humic acids.
Soil solution DOM is typically collected by lysimeters, though reverse osmosis may be a good technique in saturated systems. Water soluble organic matter is extracted with dilute salt solutions adjusted to the pH, ionic strength, and Na/Ca ratio of the soil. A number of techniques can be used to fractionate DOM including ultrafiltration and sorption chromatography. The use of macroreticular and ion-exchange resins to fractionate DOM into hydrophobic and hydrophilic fractions is increasing. Based on these techniques, DOM has been shown to be about 50% hydrophobic acids and neutrals. Concentrations of these fractions decrease with depth in the soil. Humic material, particularly fulvic acid, makes up a large percentage of DOM. Transient amounts of identifiable organics, such as lipids, carbohydrates, and organic acids, also make up a significant fraction of total DOM in soil solution. Characterization of WSOM by NMR spectroscopy has shown that this material is significantly different from base-extracted fulvic acid. Water soluble organic matter, and presumably soil solution DOM, have higher proportions of O-alkyl-C species typical of polysaccharides compared to base-extracted humics.

SREL Reprint #2028

Herbert, B.E. and P.M. Bertsch. 1995. Characterization of dissolved and colloidal organic matter in soil solution: A review. pp. 63-88 In: W. W. McFee and J. M. Kelly. (Eds.). Eighth North American Forest Soils Conference: Carbon Forms and Functions in Forest Soils. Soil Science Society of America, Gainesville, FL.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).