Microwave-assisted extraction and hydrolysis: An alternative tool to pyrolysis for the analysis of recalcitrant organic matter? Application to a forest soil (Landes de Gascogne, France)

Abstract : A comparison was made between the composition of the recalcitrant organic matter (ROM) isolated from a sandy forest soil as revealed by microwave assisted extractions and/or hydrolyses and by usual pyrolysis techniques. Successive microwave irradiation treatments were performed in H2O, 0.1 and 1 M HCl and 0.1 and 1 M KOH. At each step of the treatment the insoluble residue was examined via Curie point pyrolysis (CuPy) and Curie point thermally assisted hydrolysis and methylation (CuTHM). Sequential irradiation treatments resulted in ca 35% degradation of the ROM. Compounds released on microwave irradiation in H2O and in HCl were dominated by glucose, suggesting the occurrence of carbohydrate-containing molecular associations in the soil organic matter (SOM) which were not disrupted during acid hydrolyses and extractions as applied for the isolation of the ROM. The product distribution from the microwave irradiation in KOH showed an important contribution to the ROM from the higher plant polyesters cutin and suberin, and to a lesser extent from lignin. Different lignin-derived compounds were specifically released upon microwave acid or base hydrolyses. This suggested that two types of lignin monomers, ether or ester linked, occurred in the ROM. The changes observed in the composition of the CuPy pyrolysates of the residues from the different microwave hydrolyses are consistent with the near complete removal of carbohydrates by microwave HCl hydrolysis. The changes observed in the composition of the CuTHM pyrolysates of the residues from the different microwave acid and base hydrolyses are in agreement with a major release of cutin- and suberin-derived compounds upon microwave KOH hydrolysis. The CuPy and CuTHM pyrolysates of the final residue consists predominantly in lignin-derived compounds. This study emphasizes the potential of microwave-assisted hydrolyses to give a better estimate of the actual contribution of cutin to the ROM. However, this technique appears to be unable to completely release the lignin-based constituent of the ROM. Microwave irradiation appears to provide great potential as a tool for extraction and chemical characterization of complex OM and could be an attractive additional technique to pyrolyses.