Degradation of synthetic 14C-lignin by various white-rot fungi in soil

We investigated the degradation of synthetic lignin (14C-ring-labeled DHP) by various white-rot fungi in soil. Soil contained lignocellulose substrate (wheat straw), and the mixture was autoclaved before inoculation. The fungal species were Abortiporus biennis, Bjerkandera adusta, Dichomitus squalens, Phanerochaete chrysosporium, Phanerochaete sordida, Phlebia radiata, Pleurotus ostreatus, Trametes hirsuta, and Trametes versicolor. All fungi were incubated for 56 days with a straw-to-soil ratio of 1:5, and T. hirsuta was also cultured on straw without soil. In addition, B. adusta and T. hirsuta were studied with three other straw-to-soil ratios (1:2.5; 1:10; and 1:25). 14CO2 was collected during incubation, and after incubation cultures were successively extracted with water, dioxane, and alkali, and the residue was combusted. Mineralization of 14C-DHP by fungal inocula in soil was 4% with P. sordida and 23% with T. versicolor. Without soil T. hirsuta mineralized 30% of 14C-DHP, while the mineralization in autoclaved controls was only 0.2%. As the amount of straw decreased, fungi grew less readily, and T. hirsuta mineralized smaller amounts of 14C-DHP to 14CO2. However, the mineralization by B. adusta did not alter significantly until the straw-to-soil ratio was as low as 1:25. The dioxane fraction, representing unmodified 14C-DHP, decreased significantly during incubation of fungi with all straw treatments. By day 0, 62–64% of 14C-DHP was already bound to humic substances without fungal activity, but the mass balance of radiolabel in controls remained unchanged in spite of the incubation. White-rot fungi were apparently able to degrade the humic bound lignin, because the amount of radiolabel in humic fractions decreased in the course of incubation. However, most of 14C-DHP remained bound to humic substances, and the binding increased as the amount of straw decreased.