Lignin: A Likely Precursor of a Significant Fraction of Humic Substances Via Oxidative Transformations

Lignin is a major component of decaying terrestrial vegetation in soils and has been arguably reported to contribute substantially to the formation of soil carbon humus, and natural dissolved organic matter (DOM). To better understand the process by which this humification occurs, lignin and lignin-derived monomers were subjected to both biotic and abiotic oxidation processes. Two well-known oxidative transformation strategies were employed. The first involved the fungal degradation of brown-rot degraded wood subjected to a white-rot fungus (Phanerochaete chrysosporium) whose enzymes are particularly effective in lignin degradation via enzymatic oxidation. This enzymatic attack was monitored by ultrahigh resolution mass spectrometry (Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, ESI-FTICR-MS) of water-soluble extracts of the fungal cultures. The molecular level characterizations showed that the P. chrysosporium fungi induced aromatic ring oxidations followed by ring opening, as expected. However, the production of new molecules, some of which are aliphatic, was also observed. In addition to the water-soluble extracts of the fungal cultures, humic acids (HA) extracted from water insoluble fraction of fungal degradation of lignin, were studied. Both Fourier transform infrared spectroscopy (FTIR) and ESI-FTICR-MS results showed that fungal degradation, an integral part of the soil humification process, transformed lignin-derived aromatic molecules and simultaneously created new aliphatic molecules. Our results strongly suggest that humification by white-rot fungi in soil transforms lignin to HS with a predominant aliphatic character. Secondly, to study the fate of lignin monomers known to be produced through fungal degradation of lignin, catechol, was chosen and was subjected to hydroxyl radical (•OH) degradation, another well-known oxidation process occurring in soils and natural waters. Multidimensional nuclear magnetic resonance spectroscopy (NMR) and ultrahigh resolution mass spectrometry results show that catechol is drastically transformed to a variety of different molecules including phenolic compounds, olefins, polyols (sugar-like alcohols), and carboxyl-rich alicyclic molecules (CRAM). Results of these two studies suggest that lignin can potentially form new molecules that are not traditionally recognized as being derived from lignin and can play a much more important role in the global production of HS including soil organic matter (SOM) and DOM than previously thought