Humification, related

J.C. Vedy and S. Bruckert, Seasonal evolution of soluble organic compounds in relation to different biochemical humification processes, Pedologie 20 (1970) 135-152. 

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). 

Haider, K. (1992). Problems related to the humification processes in soils of temperate climates. In Soil Biochemistry, Vol. 7, Stozky, G., and Bollag, J.-M., eds., Marcel Dekker, New York, pp. 55-94. 

The ultraviolet-visible (UV-Vis) spectra of FIAs and FAs are somewhat featureless in showing a continuous increase of absorbance with decreasing wavelength. The absence of any well-defined UV-vis maxima and minima feasibly results from extended overlap of absorbances of a wide variety of chromophores affected by various substitutions (Stevenson, 1994 Senesi and Loffredo, 1999). Despite these limitations, the ratio of absorbances at 465 nm and 665 nm, referred to as the EJE6 ratio, has been found to vary with the nature of HS and has been widely used for characterization purposes (Stevenson, 1994 Senesi and Loffredo, 1999). In particular, the E4/E6 ratio appears to be inversely related to the MW and the degree of condensation of aromatic constituents of HS and is considered as an index of humification (Kononova, 1966 Chen et al., 1977). 

A spectroscopically demonstrated molecular property of SOM relating to the degree of humification is the SFR concentration, as measured by EPR (Riffaldi and Schnitzer, 1972 Schnitzer and Levesque, 1979 Martin-Neto et al., 1991 Senesi et al., 1996 Jerzykiewicz et al., 1999 Watanabe et al., 2005). The content of paramagnetic species is proportional to the EPR spectrum area that can be obtained by double integration of the first derivative EPR spectrum, which is normally detected. An approximation commonly used to obtain the relative area of free radicals is the... 

The effect of montmorillonite and kaolin, saturated with calcium, aluminium, or cupric ions, as well as quartz, on humic-like substances formed from glucose-tyrosine was examined by Arfaioli et al.541 All systems promoted their formation, the effectiveness being strictly related to the amount of added cation. Humification appeared to be due more to the cations than to the type of clay mineral. The clayey systems gave more complex (aromatic) substances than the quartz ones. The cations seemed more effective when free, i.e., associated with quartz rather than with the clays. The nature of the cation was also important, cupric being the most active here. In the end, all systems took on a deep dark colour. 

Aaby B. and Tauber H. (1975) Rates of peat formation in relation to degree of humification and local environment, as shown by smdies of a raised bog in Denmark. Boreas 4, 1-17. 

Volk and Schnitzer (1973) concluded that variations in the functional group components and spectral properties of humic acids from a group of Florida mucks indicated that higher rates of humification were related to (1) greater amounts of carboxyl, phenolic hydroxyl, quinone, and ketonic carbonyl groups (2) fewer alcoholic hydroxyl groups and aliphatic structures, as per IR evidence and (3) increments in EJEf, ratios and free-radical contents as revealed by ESR spectroscopy (Table 12). 

These features for lake sediment humic acids suggest that they are closely related to their precursory materials (e.g., phytoplankton) and show a relatively low degree of humification. 

Schnitzer, M. and Desjardins, J. G. (1965). Carboxyl and phenolic hydroxyl groups in some organic soils and their relation to the degree of humification. Can. J. Soil Sci. 45,257-264. 

In summary, die conibination of aging eiqieriments, isotopic enrichment, and stracture/dynamic characterization provided molecular-level information on humification processes in relation to metal mobility and binding that has eluded scientific inquiries in the past. We believe that much more novel mechanistic findings are yet to be uncovered for metal stabilization purposes, by continuing and expanding this approach. 

---