This 1962 study examined lignin decomposition by soil bacteria and chemical transformations of lignin. Researchers isolated aerobic, Gram-negative bacteria belonging to Pseudomonas and Flavobacterium from soil using native lignin prepared via the Brauns method. These bacteria decomposed 20-30% of lignin added to culture media. Notably, the study identified two decomposition processes: biological decomposition occurring during active incubation, and non-biological autoxidation occurring during desiccation of the medium. Autoxidized lignin products were dark brown, water-soluble but ethanol-insoluble compounds that readily formed complexes with nitrogen compounds from peptone. These complexes contained approximately 2.5% nitrogen, with about half as α-amino nitrogen. The researchers systematically analyzed residual lignin and oxidized lignin fractions, detecting amino acids and sugars in hydrolysates through paper chromatography. The findings suggest a connection between these lignin-nitrogen complexes and soil humic acids, indicating that non-biological chemical reactions may contribute significantly to organic matter transformation in soil alongside microbial processes.

Key findings

• Pseudomonas and Flavobacterium species isolated from soil can biologically decompose native lignin, with mixed cultures degrading approximately 80% of added lignin over 6-8 weeks
• Non-biological autoxidation of lignin occurs during medium desiccation, producing water-soluble, dark brown compounds independent of microbial activity
• Autoxidized lignin products form nitrogen-containing complexes with peptone containing approximately 2.5% nitrogen, with ~50% as α-amino nitrogen
• Amino acids and sugars were detected in acid hydrolysates of both original and decomposed lignin samples via paper chromatography
• Lignin-nitrogen complexes formed during autoxidation may be precursors to soil humic acids