Humification, degree

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

Figure 16.39. Data showing correlation among fluorescence methods to determine humification degree of HA. A4AX is the humification index proposed by Zsolnay et al. (1999) it is calculated through the ratio between areas of the upper quarter of emission spectra (435-480nm) and the lower quarter (30( M45 nm) when excitation is made at 240nm. Iv is the humification index proposed by Kalbitz et al. (1999) it is calculated through the ratio of peak intensities in 465 and 399 nm measured in fluorescence synchronous-scan excitation spectra. A46S is the humification index proposed by Milori et al. (2002) it is calculated by fluorescence area of emission spectra when the excitation is made at 465 nm.

Figure 16.42 illustrates the spectra of a whole soil sample and of the same calcinated soil. As seen, the soil fluorescent emission is basically due to organic matter (OM) (Milori et al., 2006). As the soil samples are illuminated with near-ultraviolet or blue light, mainly the more complex structures are excited, and the area under the normalized curve (divided by C content) provides a parameter proportional to the humification degree of SOM. 

Figure 16.45. SOM humification degree obtained through LIF spectroscopy (//LIF) as affected by land use and soil management system in the experiment at Brazilian Cerrado (Milori et al., 2006).

Bayer, C., Mielniczuk, J., Martin-Neto, L., and Ernani, P. R. (2002b). Stocks and humification degree of organic matter fractions as affected by no-tillage on a subtropical soil. Plant and Soil 238(1), 133-140. 

Milori, D. M. B. E, Martin-Neto, L., Bayer, C., Mielniczuk, I, and Bagnato, V. S. (2002). Humification degree of soil humic acid determined by fluorescence spectroscopy. Soil Sci. 167(11), 739-749. 

The determination of the methoxy group (OCH ) content of soils is important in studies concerned with the degree of humification of soil organic matter. Methods used to determine methoxy groups in soils have generally been based on volumetric modifications of the classical Zeisel method, which is a complicated and tedious procedure requiring specialised apparatus. 

A number of indexes based on the distribution of organic C in humic and nonhumic fractions are used for the evaluation of the humification level in organic amendments and amended soils (Senesi, 1989). Among others these include the degree of humification (DH), which is calculated as DH% = 100 x (HAC + FAC)/ TEC the humification rate (HR), which is calculated as HR% = 100 x (HAC + FAC)/ TOC and the humification index (HI), which is calculated as HI = NHC/ (HAC + FAC) (Sequi et al., 1986 Ciavatta et al., 1988). TOC represents the total organic carbon in the sample TEC represents the total extractable C by an alkaline solution (NaOH and/or Na4P207) HAC and FAC represent the C content in HA and FA fractions, respectively and NHC represents the nonhumified C content calculated by difference, that is, NHC = TEC - (HAC + FAC). 

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). 

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. 

De Nobili, M., and Petrussi,F. (1988). Humification index (HI) as evaluation of the stabilization degree during composting. I. Ferment. Technol. 66,577-583. 

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... 

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... 

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. 

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. 

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