Microwave assisted extraction and hydrolysis: An alternative 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 with microwave assisted extractions and/or hydrolysis, and using common pyrolysis techniques. Successive microwave irradiation treatments were performed in H2O, 0.1 and 1M HCl and 0.1 and 1M KOH. At each step the insoluble residue was examined via Curie point pyrolysis (CuPy) and Curie point thermally assisted hydrolysis and methylation (CuTHM). Sequential irradiation treatment 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 hydrolysis and extraction 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 hydrolysis. This suggested that two types of lignin monomers, ether- or ester-linked, occurred in the ROM. The changes 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 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 consist predominantly of lignin-derived compounds. The study emphasizes the potential of microwave assisted hydrolysis to give a better estimate of the actual contribution of cutin to ROM than pyrolysis. However, the 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 characterisation of complex OM and could be an attractive additional technique to pyrolysis.