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8 9 A comparison was made between the composition of the recalcitrant organic matter (ROM) 10 isolated from a sandy forest soil as revealed by microwave assisted extractions and/or 11 hydrolyses and by usual pyrolysis techniques. Successive microwave irradiation treatments 12 were performed in H2O, 0.1 and 1 M HCl and 0.1 and 1 M KOH. At each step of the 13 treatment the insoluble residue was examined via Curie point pyrolysis (CuPy) and Curie 14 point thermally assisted hydrolysis and methylation (CuTHM). Sequential irradiation 15 treatments resulted in ca 35% degradation of the ROM. Compounds released on microwave 16 irradiation in H2O and in HCl were dominated by glucose, suggesting the occurrence of 17 carbohydrate-containing molecular associations in the soil organic matter (SOM) which were 18 not disrupted during acid hydrolyses and extractions as applied for the isolation of the ROM. 19 The product distribution from the microwave irradiation in KOH showed an important 20 contribution to the ROM from the higher plant polyesters cutin and suberin, and to a lesser 21 extent from lignin. Different lignin-derived compounds were specifically released upon 22 microwave acid or base hydrolyses. This suggested that two types of lignin monomers, ether 23 or ester linked, occurred in the ROM. The changes observed in the composition of the CuPy 24 pyrolysates of the residues from the different microwave hydrolyses are consistent with the 25 near complete removal of carbohydrates by microwave HCl hydrolysis. The changes observed 26 in the composition of the CuTHM pyrolysates of the residues from the different microwave 27 acid and base hydrolyses are in agreement with a major release of cutinand suberin-derived 28 compounds upon microwave KOH hydrolysis. The CuPy and CuTHM pyrolysates of the final 29 residue consists predominantly in lignin-derived compounds. This study emphasizes the 30 potential of microwave-assisted hydrolyses to give a better estimate of the actual contribution 31 * Corresponding Author Tel. +33 1 4427 6715 ; fax : +33 1 4329 5102 E-mail address : [email protected] (B. Allard). bi oe m co -0 04 55 79 3, v er si on 1 11 F eb 2 01 0 Author manuscript, published in "Organic Geochemistry 40 (2009) 1005-1017"