Effect of Land Management on Humic Acid Spectra Generated Using 13c-NMR Spec

Organic matter in soils and waters is conveniently categorized as humic and non-humic substances based on operational definitions. Humic substances are organic materials derived principally from decaying plant remains but with the normal plant components considerably altered by the soil animal, and microbial populations. Abiological chemical reactions are also possible the entire process giving rise to a complex mixture of macromolecules whose composition is site-dependent, especially on vegetation and pedogenic processes. Since Achards first extraction of humic substances in 1786, the use of different isolation and fractionation techniques has led to considerable confusion until in 1982, when the International Humic Substances Society published methodologies for the extraction and fractionation of humic substances, from both soils and waters. The percentage distribution of humic fulvic and hydrophilic acids has been fully described by Anderson et al. (1984). Humic substances are the most abundant of the naturally occurring OM in well humified soils. From cross polarization magic angle spinning (CPMAS) 13C nuclear Magnetic Resonance (NMR) Spectroscopy and from sugar and amino acid analysis of humic substances it has been found that each humic component in each environment possesses an individuality that distinguishes it from other components in the same environment and from the same humic components in different environments. There is no evidence to suggest that there is genetic or biological control of the synthesis of humic substances. Hence they lack regularity in the sequencing of the molecules which compose the macromolecules. Further intramolecular and intermolecular associations can take place within and between humic (and other) macromolecules to give a semblance of secondary and tertiary structure. However, there are no rigid regularities between such associations. Although associations can form between humic substances and other organic and inorganic materials such associations are invariably random, and are not necessarily a part of any biological function which humic substances may have. Hence humic substances do not meet any of the criteria for structure as they apply for proteins. Fractionations of the humic substances on the basis of molecular size differences are most frequently used to further analyse the humic substances. The procedures generally involve uses of gel permeation chromatography, ultrafiltration or centrifugation. For gel permeation and ultrafiltration procedures to be effective it is important that the humic substances should not interact (either adsorb to or be rejected by the gel or membrane) with the media used. Appelqvist (1990) fractionated sodium humate preparations using ultrafiltration (with Sartorius membranes of pore sizes with nominal molecular size exclusion values of 5,000, 20,000 and 100,000 daltons) and gel chromatography (using Sephacryl S-200 gel, a cross-linked dextran from Pharmacia). Differences were observed between the samples excluded by the gel (MW > 150,000) and those retained by the membranes which nominally retained materials of 100,000 MW and above (> 100,000), 20,000 and above (100,000 - 20,000 MW fraction) and 5,000 and above (20,000 - 5,000). As the molecular sizes decreased, the carbon contents also decreased (52.5, 51.6, 49.2, 47.4%) and the E4/E6 ratios increased. The E4 (solvent mixture absorbance at 465nm) and E6 (solvent mixture absorbance at 665nm) are indicators of differences in solution conformations. It aids comparison of humic substances in different solvent systems. Such data support the concept of greater aromaticity, for the higher molecular weight materials. This is in keeping with the concept of greater numbers of aromatic carboxylic acids, in the higher molecular weight substances. Hence, this book is all about the effect of land management on humic acid spectra generated using 13C-NMR spectroscopy.