Humification humic substances

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. 

Labile and refractory DOM undergo abiotic photochemical reactions in the photic zone, especially in the sea surfece microlayer where physical processes concentrate DOM into thin films. Some of these reactions appear to be important in the formation of refractory DOM and others in its degradation. For example, DOM exuded by diatoms during plankton blooms has been observed to be transformed into humic substances within days of release into surfece seawater. Laboratory experiments conducted in seawater have demonstrated that photolysis of labile LMW DOM promotes the chemical reactions involved in humification and produces chemical structures foimd in marine humic substances. 

Humification The formation of humic substances via abioUc reacUons, including condensations and... 

Soil and related environments are both an important natural habitat of biota and a natural reservoir of biotic debris consisting of plant remains and dead animals and microorganisms. With time, dead remains are subject to continuous turnover, either mineralized or transformed to diverse organic components which are termed humus. This process is referred to as humification. Humus is composed of humic substances plus nonhumic substances that have become stabilized and are thus an integral part of soil and related environments (Table 2.1). 

PATHWAYS OF HUMIC SUBSTANCE FORMATION 2.4.1. Selective Preservation Pathways of Humification... 

Smectites Condensation of arginine and glucose at 37 °C C and N fractions yields FTIR spectra of solid residue and supernatant humic substances XRD investigation into change in d-spacings Integrated Abiotic Humification Pathway Gonzalez and Laird (2004)... 

Figure 2.18. Proposed mechanism for the catalysis of hydroxylaluminosilicate ions in catechol humification. Reprinted from Liu, C., and Huang, P. M. (2002). Role of hydroxylaluminosilicate ions (proto-imogolite soil) in the formation of humic substances. Org. Geochem. 33, 295-305, with permission from Elsevier.

In the past the mineral matrix was considered as inert, only providing stabilization support for enzymes and humic substances however, due to the overwhelming amount of evidence at the molecular level, there is no doubt that minerals participate in abiotic catalysis of humification reactions in soils. Naidja et al. (2000) referred to mineral particles as the Hidden Half of enzyme-clay complexes, which not only prolong the activity of immobilized enzymes but also are readily able to participate in electron transfer reactions. Many environmental factors can negatively affect the... 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Liu, C., and Huang, P. M. (2000). Catalytic effects of hydroxyl-aluminum and silicic acid on catechol humification. In Humic Substances Versatile Components of Plants, Soil and Water, Ghabbour, E., and Davies, G., eds., Royal Society of Chemistry, Cambridge, UK, pp. 37-51. 

SOM, soil organic matter HS, humic substances DH, degree of humification HAC, humic acid C FAC, fulvic acid C TEC, total extractable C HR, humification rate HI, humification index NHC, nonhumified C TOC, total organic C HA, humic acid FA, fulvic acids UV-Vis, ultraviolet-visible FT-IR, Fourier transform infrared NMR, nuclear magnetic resonance ESR, electron spin resonance EEM, excitation-emission matrix. 

Hence, these results can be interpreted as supportive of the hypothesis that changes in molecular weight within bulk humic material are indicative of evolution along the path of humification. In other words, fractionation of operationally defined humic substances in accordance with size allows for separation of fractions with different degrees of humification The smaller the fractions, the more they have... 

Govi, M., Ciavatta, C., and Gessa, C. (1994). Evaluation of the stability of the organic matter in slurries, sludges and composts using humification parameters and isoelectric focusing. In Humic Substances in the Global Environment and Implications on Human Health, Senesi, N., and Miano,T. M., eds., Amsterdam, 1311-1316. 

Determination of Humification Degree of Soil Organic Matter and Humic Substances in Different Ecosystems,... 

Rosa, A. H., Simoes, M. L., De Oliveira, L. C., Rocha, J. C., Martin-Neto, L., and Milori, D. M. B. P. (2005). Multimethod study of degree humification of humic substances extracted from different tropic soils profiles in Brazil s Amazonian region. Geoderma 127(1-2), 1-10. 

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