A comparative study on the chemical composition of humic acids from forest soil, agricultural soil and lignite deposit - Bound lipid, carbohydrate and amino acid distributions

Base- and acid-hydrolysable fractions of humic acids (HAs) isolated from a forest soil, an agricultural soil and a lignite deposit were analysed, and comparisons were made between the base hydrolysable lipid (bound lipid), carbohydrate and Al amino acid signatures of the different humic acids. Bound lipids differ depending on the humic acid origin. Their composition were rather similar for the two soil humic acids, with three main lipid classes identified: (i) aliphatic components, (ii) aromatic components and (iii) sterols and triterpenols. The aliphatic subtraction was the most abundant and consisted predominantly of cutin- and suberin-derived moieties some of which could be clearly related to the vegetation. A minor bacterial input was indicated by the presence of short chain alpha- and beta-hydroxyalkanoic acids. Aromatic subfraction contributed to a low amount to the total base hydrolysates and consisted mainly of lignin-derived methoxyphenols. Present in trace amounts, sterols and triterpenols are mainly of higher plant origin. The base hydrolysate from lignite humic acid markedly differs. Bound lipids released from lignite HA comprised almost exclusively aliphatic components, largely dominated by long chain alkanoic acids. Lignin-derived moieties, hardly detected, consisted solely of vanillic and 4-hydroxybenzoic acids indicating a much higher degree of lignin alteration in lignite humic acid. Sterols and triterpenols were absent. Although the composition of monosaccharides released upon acid hydrolysis was rather uniform irrespective of the humic acid origin, the distribution changed with the degree of humification of the HAs. Ratios of (Galactose+Mannose) to (Xylose+Arabinose) increased from soil to lignite humic acids. The high values of the ratios indicate that carbohydrates are primarily of microbial origin. In all humic acids neutral and acidic protein amino acids dominated. Non protein amino acids were only minor components consisting mainly of hydroxy proline and ornithine. The amino acid distributions of both soil HAs were similar. The amino acid distribution of lignite HA resembled that of soil HAs except for the following differences: (1) the absence of hydroxy proline and the greater abundance of ornithine suggesting a higher microbial contribution to the amino acids as the degree of humification increases, (2) the higher contribution of polar amino acids suggesting a preferential preservation of these amino acids possibly by interaction with the humic acid surface through hydrogen bonds. (c) 2005 Elsevier B.V. All rights reserved.