Fulvic acid in soil

As a result of microbial formation of metal-organic complexes with fulvic acids in soils of Tropical Rain Forest ecosystems, the surface and sub-surface runoff waters are enriched in some heavy metals like manganese and copper. A similar tendency has been shown for boron, strontium and fluorine. 

Spectrofluorimetric methods are applicable to the determination of aliphatic hydrocarbons, and humic and fulvic acids in soil, aliphatic hydrocarbons polyaromatic hydrocarbons, optical whiteners, and selenium in non-saline sediments, aliphatic aromatic and polyaromatic hydrocarbons and humic and fulvic acids in saline sediments. The only application found in luminescence spectroscopy is the determination of polychlorobiphenyl in soil. Generally speaking, concentrations down to the picogram (pg L 1), level can be determined by this technique with recovery efficiencies near f00%. 

NMR has been applied to the determination of organomercury compounds in non-saline sediments and humic and fulvic acids in soil and saline sediments. 

Humic and fulvic acids contain various types of phenolic and carboxylic functional (hydrophilic) groups as well as aromatic and aliphatic moieties which import hydro-phobic properties to these substances. Fig. 4.12 gives a schematic idea on the composition of these substances. We refer to the book of Thurman (1985) and Aiken et al. (1985) for a description of the various properties of humic and fulvic acids in soils and waters and the book by Buffle (1988) for the coordinating properties of humus and humic acids. 

Considering the pH range 4—10 as the geochemical range, the redox potential due to the breakdown of water due to redox processes, the rare earths are predominantly present as Ln(III). Since the anions OH- and CO2 are present in natural environments, rare earths combine with these anions to form insoluble hydroxides and carbonates and hence immobilized. At lower pH, rare earth ions are adsorbed on clays, which are natural ion exchangers. The interactions of rare earth ions with humic and fulvic acids in soils, and Fe/Mn oxides are so strong, that they become immobile. 

Humus acids are a main component extracted by water from organic matter of rocks and deposits, first of all from soil and peat. The content of humic acids reaches in soils 10% by mass, in peats 25-50%, in coals up to 60%. The contents of fulvic acids - up to 15% in peats and 60% in coals. Relative contents of humic and fulvic acids in soils depend on their properties. In tundra and bleached forest soils dominate fulvic and in peats, chestnut and black-earth soils, humic acids. Frequently (but not always), with depth fraction of fulvic acids increases. 

Despite the advances made in high-performance liquid chromatography in recent years, there are still occasionally applications in which conventional column chromatography is employed. These methods lack the sensitivity, resolution and automation of HPLC. They include the determination of urea herbicides in soil, polyaromatic hydrocarbons, carbohydrates, chloroaliphatic compounds and humic and fulvic acids in non-saline sediments. The technique has also been applied in sludge analysis, e.g. aliphatic hydrocarbons and carboxylic acids. 

The Suwannee River was sampled at its origin at the outlet of the Okefenokee Swamp. This fulvic acid, therefore, is likely to be less degraded than a sample from the Calcasieu River that was taken near its mouth on the estuary during a warm, low-flow period in early summer. Metal-ion solubility controls and sorption on mineral surfaces in upland soils also might fractionate the fulvic acid in the Calcasieu River. In contrast, the Suwannee River mineral-soil solubility controls are less significant. 

Schnitzer, M., and E. H. Hansen. 1970. Organo-metallic interactions in soils. 8. An evaluation of methods for the determination of stability constants of metal-fulvic acid complexes. Soil Science 109 333-340. 

Sposito, G., Holtzclaw, K. M., LeVesque, C. S., and Johnston, C.T. (1982). Trace metal chemistry in arid-zone field soils amended with sewage sludge II. Comparative study of the fulvic acid fraction. Soil Sci. Soc. Am. I. 46, 265-270. 

Dissolved organic carbon content does, however, appear to influence the release of atrazine from soil (Clay and Koskinen, 1990a Liu et al., 1995), with more released in the presence of dissolved organic content. Fulvic acid in... 

Sarkar JM, Bollag JM, Malcolm RL (1988) Enzymatic coupling of 2, 4-dichlorophenol to stream fulvic acid in the presence of oxidoreductases. Soil Sci Soc Am J 52 688-694... 

Schnitzer, M. Hansen, E. H. Organo-metallic Interactions in Soils 8. An Evaluation of Methods for the Determination of Stability Constants of Metal-Fulvic Acid Complexes, Soil. Sci. 1970, 109, 333-340. 

Antweiler R. C. (1991) The hydrolysis of Suwannee River fulvic acid. In Organic Substances and Sediments in Water Volume I. Humics and Soils (ed. R. A. Baker). Lewis Publishers, Chelsea, MI, pp. 163-177. 

The above scheme provides further fractionation of humic acids allowing the separation of a fraction known as hymatomelanic acid and of fulvic acids in several fractions (B, D, etc.). Humin is commonly further purified of inorganic components. The elemental composition of the fractions obtained with this separation scheme from an uncultivated prairie brown soil (TypicXerochrept) is given in Table 14.1.1. 

Figure 1. Fluorescence enhancement titration curve for 15.0 mg/L soil fulvic acid in 0.1 M NaC104 at pH 4.00 and 25 °C (O) measured fluorescence. Line is SigmaPlot fitted curve excluding last titration point. The fluorescence is monitored at an excitation wavelength of 360 nm and an emission wavelength of420 nm.

The absolute amounts of aniline covalently bonded to the soil fulvic acid in the presence and absence of the peroxidase were not measured. However, the relative signal to noise ratios obtained in the NMR spectra indicate that significantly more aniline was taken up by the fulvic acid in the enzyme catalyzed reaction. It should also be pointed out that, in the execution of the peroxidase experiment, the solution containing the fulvic acid, aniline, and peroxidase darkened instantaneously upon addition of the hydrogen peroxide, indicating significantly faster kinetics than in the nonenzyme reaction. 

Humic substances in soil have properties similar to those found in other natural systems. Accordingly, results obtained by the soil scientist on the nature, origin, and reactions of these complex substances are of considerable interest to researchers in several disciplines of science. In this chapter emphasis is given to historical aspects of research on soil humic substances, to the interactions of humic and fulvic acids with soil mineral components, and to the participation of humic substances in geochemical processes. 

Despite the current popularity of the polyphenol theory, a completely satisfactory scheme for the occurrence of humic and fulvic acids in diverse geologic environments has yet to be established. In practice, all pathways may be operative, but not to the same extent in all environments or in the same order of importance. A lignin pathway may-predominate in wet sediments, such as peats and swamps. The drastic conditions existing in soils under a harsh continental climate (e.g., some Mollisols) may favor humus synthesis by sugar-amine condensation. The disappearance of amino acids from buried sediments has been attributed to the formation of brown nitrogenous polyelectrolytes by reaction with reducing sugars (Stevenson, 1974). 

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