Humic substances functions

In some systems, such as lake and river waters, the suspended inorganic particles may be coated by biological polymers, termed humic substances, which prevent flocculation by either steric or electrostatic mechanisms. These can also interact with added inorganic salts (31) that can neutralize charged functional groups on these polymers. 

This chapter focuses on the effects of humic substances present at the rhizo-sphere on plant growth and nutrient uptake. The main structural features of humic substances, their nutritional function, and the capacity to interact with plant metabolism are also presented. 

Humic substances can form complexes with metals, including cationic micronutrients (36), thanks to the presence of electron-donor functional groups in these molecules. It therefore appears evident that due to these properties, humic substances can contribute to the regulation of the chemical balances of metals, thus influencing their solubility (5). With regard to plant availability, the molecular dimension and solubility of humic substances are very important. 

Much of the Hgt pool was found in the upper part of the soil, which is rich in organic matter. This pattern is likely due to an elevated atmospheric deposition of Hgt over the extended period and immobilization of mercury by organic functional groups and accumulation of organic matter as part of the soil-forming process. The retention of mercury in the mor humus layer was almost complete due to the very strong association between Hgt and humic substances. 

Hosse, M. and Wilkinson, K. J. (2002). Determination of electrophoretic mobilities and hydrodynamic radii of three humic substances as a function of pH and ionic strength, Environ. Sci. Technol., 35, 4301-4306. 

Fig. 10. Functional groups in humic substances from 11 soil samples (mol C/kg) (data were taken from Zelazny and Carlisle [56])...

As with the bulk POM and DOM, the operationally defined fractions of UDOM and humic substances are quantified by elemental analysis and via broad molecular-class detection. Other strategies involve measurement of the natural isotopic composition, both stable and radioactive, of the various fractions. Efforts are underway to develop more sophisticated techniques, such as solid-state NMR and high-resolution mass spectrometry, far identification of specific bonds and functional groups. 

The basic structure of humic substances involves a backbone composed of alkyl or aromatic units crosslinked mainly by oxygen and nitrogen groups. Major functional groups attached to the backbone are carboxylic acids, phenolic hydroxyls, alcoholic hydroxyls, ketones, and quinones. The molecular structure is variable as it is dependent on the collection of DOM available in seawater to undergo the various polymerization, condensation, and oxidation reactions and reaction conditions involved in humification, as well as the ambient physicochemical reaction conditions, such as temperature and light availability. 

The capacity of complexing of humic substances is ascribed to their oxygen-based functional group (table 8.19). 

Figure 8.26 (A) Functional groups of humic substances. (B) Complexing schemes for fulvic substances. Reprinted from Stevenson (1983), with kind permission of Theophrastus Publishing and Proprietary Co.

In addition to their complexing capability, humic substances can also reduce oxidized forms of metal cations and polyanions (Fe —> Fe MoO —> Mo " VO3 —> VO see Szilagyi, 1971). The reduced forms are then embodied in the structure of humic substances by functional-group fixation. 

P.A. Cranwell and R.D. Haworth, The chemical nature of humic acids, in D. Povoledo and H.L. Golterman (Eds.), Humic Substances Their Structure and Function in the Biosphere, Centre of Agricultural Publishing and Documentation, Wageningen, 1975, pp. 13-18. 

Perdue, E. M. 1985. Acidic functional groups of humic substances. In Humic Substances in Soil, Sediment and Water Geochemistry, Isolation, and Characterization, (G. R. Aiken,... 

Note Humic substances (HS) are operationally defined as DOC that is retained on XAD-2 or XAD-8 resins. UDOM is operationally defined as DOC that is retained by a membrane with a 1 nm pore size and 1000 Da molecular weight cutoff. 13C-NMR spectroscopy was used for analysis of carbon functional groups, which are presented as a percentage of the total organic carbon in the sample. THNS, total hydrolyzable neutral sugars THAA, total hydrolyzable amino acids. 

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