In humin

Lichtfouse E, Leblond C, DaSilva M, Behar F (1998b) Occurrence of biomarkers and straight chain biopolymers in humin implication for the origin of soil organic matter. Naturwissenschaften 85 497-501... 

Solid-state NMR has done much to dispel the mysteries of humin compositions, and significant advances have recently been made using proton NMR in the liquid state (see Section 15.3.3 of Chapter 15). Based on solid-state 13C NMR spectra, Hatcher et al. (1980) concluded that a repeating aliphatic structural unit, possibly attributable to branched and cross-linked algal or microbial lipids, is common to both soil and sediment humin samples. Hatcher et al. (1983) viewed the increase in humin relative to the other humic fractions as a selective preservation of the aliphatic compounds of the sediments and did not support condensation theories. 

Soil polysaccharides can amount to as much as 20% of the humic fractions isolated in aqueous media, and identification of the classes of components in humin materials in associations with the soil clays indicates that carbohydrates contribute significantly to those isolated in the DMS0/H2S04 medium (see Sections 1.4.7 and... 

By using methods analogous to those in the above studies Zelazny and Carlisle (1974) found that oxygen-containing functional group levels in humin and humic and fulvic acids from all series and layers of the Florida mucks were similar (Table 13), irrespective of their extents of humification or rates of subsidence. 

Fatty acids in humin from Lake Haruna sediments were analyzed for the fraction obtained by solvent (benzene/methanol 6 4) extraction followed by saponification 2N KOH aqueous solution at 200°C for 3 hours) extraction (Yamamoto and Ishiwatari, 1981). The fatty acids were composed of normal C12-C30 saturated monocarboxylic acids (maximum at Cig), unsaturated (Cie and Cis), and branched (C13, C15, and Cn) monocarboxylic acids. The fatty acid distribution in humin resembled that in humic acid. Total fatty acids accounted for 1.0% of the humin and probably originated from algae, bacteria, and higher plants. 

The comparison between NMR spectra of humin and humic acids in this aerobic soil bears out the fact that some basic structural differences exist between these two soil fractions. The lower polysaccharide content of humic acids compared to humin is expected. The greater relative proportion of carboxyl (or amide) carbons (peak at 175 ppm) in humic acids is another minor difference that was noted. The most important difference is the relative concentration of paraffinic carbons with humin having a much greater concentration than humic acids. Excluding the presence of polysaccharides, it is difficult to imagine that humin, in this instance, is a clay complex of humic acids. If this were the case, the spectra would be nearly identical except for the presence of carbohydrates in humin. It is also difficult to imagine that humin is a condensation product of humic acids. Rather, the comparisons show that either humic acids are decomposition products of the humin (where decomposition selectively alters the structure of individual precursors in humin), or humic acids are genetically unrelated to humin. 

On the basis of these studies on woody tissues, it seems that lignin from vascular plants can be selectively preserved compared to biologically degradable polysaccharides when buried. The same can be expected for the lignin in humin from peat the spectra shown in Figure 2 consistently demonstrate this selective preservation with increasing depth. 

By examination of the spectra in Figure 5, it is clear that polysaccharides (holocellulose, peaks at 72 and 106 ppm) are dominant in the delignified humin in the upper layers of peat but diminish in relative concentration with depth. This trend was also observed in the spectra of humin in Figure 2. At depth, the polysaccharides are minor compared to the paraffinic carbons (peak at 30 ppm). Thus, the paraffinic structures in humin are resistant to sodium chlorite oxidation, and their relative increase in concentration with... 

The occurrence of a predominantly aliphatic humin in marine sediments has been known for some time (Stadnikov, 1930 Breger, 1960 Cane, 1976 Stuermer et al., 1978). This humin was believed to be the precursor of aquatic kerogen in ancient shales and as such has been extensively studied. However, the discovery that a structurally similar aliphatic component exists in humin from peats and even soils is a major new finding that has demonstrated the usefulness of solid-state C NMR. The analytical visibility of this component in soil humic substances has been masked by the over-... 

Grasset, L., Guignard, C., and Ambles, A., Free and esterified aliphatic carboxylic acids in humin and humic acids from a peat sample as revealed by pyrolysis with tetramethylammonium hydroxide or tetraethylammonium acetate, Org. Geochem., 33, 181-188, 2002. 

---