Lignin contribution to aliphatic constituents of humic acids through fungal degradation

To investigate how fungi transform lignin in soil during humification, a naturally brown-rotted wood was subjected to additional fungal degradation by the white-rot fungus Phanerochaete chrysosporium. Both Fourier transform infrared spectroscopy and ultrahigh resolution mass spectrometry showed that fungal degradation, an integral part of the soil humification process, transformed lignin-derived aromatic molecules and simultaneously created many new aliphatic molecules. The majority of these new aliphatic molecules were chemically different from aliphatic molecules already existent in the lignin precursor extract and in the P. chrysosporium biomass. Our results strongly suggest that humification by white-rot fungi in soil transforms lignin to humic substances with a predominant aliphatic character. This challenges the concept that only aromatic structures are expected from humification of lignin by fungi. This finding also reverses current views that aliphatic constituents of soil humic materials are not principally derived from lignin.