Analytical pyrolysis and thermally assisted hydrolysis and methylation of EUROSOIL humic acid samples — A key to their source

Abstract

Humic acids have been widely investigated by spectroscopic methods, especially NMR and FTIR, and they are known to show significant differences according to their origin. Low resolution methods such as NMR and FTIR, however cannot easily distinguish different input sources or establish relations between SOM chemistry and vegetation or land use in general. High resolution methods, such as analytical pyrolysis and pyrolysis combined with methylation do offer such possibilities. Therefore, HAs from five reference soils called the Eurosoils, including a Vertic Cambisol (E1, Italy), a Rendzina (E2, Greece), a Dystic Cambisol (E3, Great Britain), an Orthic Luvisol (E4, France) and an Orthic Podzol (E5, Germany), that were previously characterized a.o. by NMR, FTIR and ESR, were also analyzed by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and thermally assisted hydrolysis and methylation (THM) and subsequent analysis by GC/MS. The Orthic Podzol sample showed the largest aliphaticity, and the strongest degradation of aliphatics and lignin. The Dystric Cambisol featured the least decomposed HA, which was reflected by a large content of long-chain alkanes, and little lignin degradation. Both the Dystric Cambisol and the Orthic Luvisol HAs contained a significant amount of microbial organic matter. Polyaromatics, which indicate the presence of charred material, were most abundant in the Vertic Cambisol and the Podzol HAs and lowest in the Dystric Cambisol and the Rendzina HAs. THM was able to distinguish between the various vegetations/land uses. Although quantifications by NMR and py-GC/MS are essentially different, the general results largely coincided. NMR appears to underestimate aromaticity and overestimate aliphaticity, but a molecular mixing model yielded reasonable correlations between NMR and pyrolysis data. Classification by ‘degradation state’ based on py-GC/MS largely coincided with acidity determined by titration, but FTIR data did not coincide. Py-GC/MS, with its much larger resolution, is a better tool to distinguish effects of vegetation, microbial input, and degradation. HA's produce the same variety of compounds upon pyrolysis as total SOM extracts and are therefore chemically not more simple than SOM. HA chemistry, however can be understood in the light of land use history and SOM dynamics.