Characterization of soil organic matter using microwave assisted acid and base hydrolysis
Résumé
Sequential microwave assisted HCl hydrolysis and tetramethylammonium hydroxide (TMAH) hydrolysis (in MeCN at 140 and 190 degrees C) were used to analyze the hydrolysable constituents of soil organic matter (SOM) from a forest soil. At each step, the organic or aqueous extracts were analyzed using gas chromatography-mass spectrometry (GC-MS) and the residues investigated using solid state C-13 nuclear magnetic resonance (NMR) spectroscopy, Curie point pyrolysis (CuPy) and thermochemolysis (CuTHM). The composition of the aqueous extract from microwave assisted HCl hydrolysis strongly depended on hydrolysis temperature and was dominated by carbohydrates at 140 degrees C and amino acids and 2-amino sugars at 190 degrees C. The C-13 NMR spectrum of the residue from acid hydrolysis at 190 degrees C was consistent with the removal of most of the carbohydrates. The organic extract was dominated by aromatic compounds, 50% of which retained the glyceryl side chain of the arylglycerol-beta-aryl ether units of lignin. However, comparison of the thermochemolysates of the residue from acid hydrolysis and of the initial SOM showed no great differences in the composition and relative abundance of aromatic lignin-derived compounds. This suggested that microwave assisted acid hydrolysis might account for only the unprotected or unaltered lignin in SOM. Thus, although more time consuming, microwave assisted acid hydrolysis provided information about hydrophilic constituents and unaltered lignin-derived constituents of SOM, which was not available from pyrolysis. Microwave assisted TMAH hydrolysis released predominantly aliphatic compounds from cutin and suberin. The release of cutin-and suberin-derived constituents from SOM upon microwave assisted base hydrolysis was supported by the C-13 NMR spectrum, CuPy pyrolysate and CuTHM thermochemolysate of the residue. (C) 2013 Elsevier Ltd. All rights reserved.