The Isolation and Characterization of Humic Substances and Humin from Grey Brown Podzolic and Gley Grassland Soils

摘要

Humic acids (HAs) and fulvic acids (FAs) were isolated by the exhaustive extractions at pH 7, 10.6, and 12.6 of a Grey Brown podzol and of a Gley soil, each in long term grassland. A further exhaustive extraction in base + 6 M urea isolated additionalHAs and FAs of significantly different compositions. A subsequent sequential extraction with dimethylsulphoxide (DMSO) + 6 M concentrated H_2S0_4 isolated humin materials. The solvent sequence used is capable of isolating > 90% of the organic matter (OM) in some soils, and at least 70% of the OM in all soils. Solid state CPMAS ~(13)C NMR spectroscopy s showed readily observable differences between the compositions of the HAs and FAs isolated at the different pH values, and by the base + urea solvent system. Humin has been considered to be the most intractable component of soil organic matter. However, applications of solid state ~(13)C NMR and of liquid state proton NMR have shown that soil humin is composed largely of biological molecules of plant and of microbial origins and with a degree of resistance to biological degradation. That resistance is enhanced by the protection afforded by close associations with the soil mineral colloids.