Abstract:

Objective: In this study, efficient cellulose-degrading fungi to degrade surface fuel of Maoer Mountain plantation were screened for investigating the effect of cellulose-degrading fungi on forest surface fuel decomposition, so as to reduce the fuel accumulations in the forest and reduce the potential risk for forest fire. Method: In this study, the fuels in Larix gmelinii, Juglans mandshurica, Fraxinus mandshurica and Picea asperata plantations were collected from Maoer Mountain Experimental Forest Station of Northeast Forestry University. The fungi were isolated and cultured using the rose bengal chloramphenicol agar medium, and then Congo red staining was used to screen strains with high cellulase activity according to the cellulose decomposition index. Morphological and molecular identification were performed. The suspensions of different strains with high cellulase activity were inoculated into flasks with three kinds of decomposition substrates (coniferous, broad-leaved and broadleaf-conifer mixed) and then cultured in a constant temperature incubator for 80 days. Samples were taken regularly, and the content of holo-cellulose was determined for analyzing the cellulose decomposition rule. The strain with the most efficient degradation effect on natural cellulose in forest fuel was selected, which was verified by scanning electron microscope. The screened efficient cellulose-degrading fungi were used to prepare the fungal disks, which were put into the malt extract medium, and the shaking culture was performed for 5 days. The fungal suspension was used as a single microbial agent, and the mixed microbial agent was obtained by mixing two fungal suspensions with equal volume. The microbial agents were sprayed into litter bags with different fuels of J. mandshurica, L. gmelinii and their mix in low, medium and high dosages. Litter bags were collected each month to determine holo-cellulose content in the fuels, and analyze the degradation effect of different microbial agents on the holo-cellulose in fuels. Result: Among fifteen fungi selected by RBC medium, eight had high cellulase activity, according to the distinct hydrolytic circles they produced on CMC-Na medium. Fungus B2 (Cladosporium ramotenellum) had the highest cellulolytic index. Fungus A4 (Sarocladium strictum) had the strongest degradation capable of holo-cellulose in the three fuel substrates based on the cellulose decomposition result, followed by fungus A2 (Peniophord incarnata). Further, the scanning electron micrographs indicated that the mycelia of isolate A4 could adhere to the leaf surface and invade the leaf tissue to degrade holo-cellulose in leaves by secreting cellulolytic enzymes. The degradation rate of holo-cellulose in the three substrates with different dosages was higher than that of the control, and the order was high concentration, medium concentration, low concentration and control. The degradation rate of holo-cellulose in the three fuel substrates with microbial agents was as follows:microbial agent C(mixed microbial agent)> microbial agent B(fungus A4)> microbial agent A(fungus A2). Conclusion: Fungus A4 is a highly effective fungus selected in this study. It also has a strong degradation ability in the field test, and the mixed microbial agent constructed with strain A2 has a better degradation effect on the holo-cellulose in the fuels. The optimal conditions of enzyme production can be explored in the follow-up study, so as to realize the degradation of forest surface fuels to a greater extent.

Key words: cellulose, fungi, screening, degradation ability, forest fuel