Abstract Gaseous metabolites including carbon dioxide are inhibitory to the growth of white-rot fungi and to lignin decomposition. Increasing the air/oxygen flow over the substrate reverses the inhibition. Lignin decomposition is more efficient in pure oxygen than in air. Organic matter and lignin losses in the initial stages of fermentation depress digestibility. The increase in digestibility of fermented substrate and loss of organic matter and lignin are correlated. By the application of pyrolysis and gas chromatography to study the changes during fungal degradation of wheat straw, 21 pyrolytic fragments were identified.