Comparison between humic substances from soil and peats using TMAH and TEAAc thermochemolysis
Résumé
Humic substances, mainly humic acids and humin, from a calcic peat (MP), an acid peat (CAL) and a calcic soil (MAT) were investigated. Important differences in the relative abundance of the various organic fractions were observed between the four samples. As shown by the HU/FA + HA ratio, the calcic peat contains the most humified organic matter, the degree of humification decreasing with pH. The organic matter in MAT was much less humified due to cultivation. Bulk information from spectroscopy showed similarities between the various humic acids and the various humins in the four samples: the presence of ester and ether groups, a marked aliphatic character and the presence of aromatic moieties. The major products of TMAH thermochemolysis were aromatic compounds derived from lignin, tannins and bark, and fatty acids of bacterial or plant origin, as the methyl esters (FAMEs). The compounds were mainly incorporated via ester or ether bonds and n-alkene/n-alkane doublets in the pyrolysate of CAL arose from resistant aliphatic biopolymers. The study of FAME distributions indicated that the bacterial contribution was higher in humic acids than in humin for all the samples. This contribution was much higher in MAT, as shown by prominent iso- and anteiso-C15 carboxylic acids and the presence of α-hydroxyacids. Bifunctional linear dicarboxylic acids were also incorporated into the humic structure (MAT and CAL), contributing to alkyl bridges. The C21–C33 n-alkanes with a Gaussian distribution found in MAT and MP samples were initially retained in the humic structure via non-covalent bonds. TEAAc thermochemolysis showed that a fraction of the aliphatic acids (with a different distribution) was not covalently bound, having been sterically trapped in the humic structure. Trapped fatty acid methyl esters were minor.