Microbial Dynamics Associated with Multiphasic Decomposition of ¹⁴C-Labeled Cellulose in Soil

Abstract

Recent emphasis on residue management in sustainable agriculture highlights the importance of elucidating the mechanisms of microbial degradation of cellulose. Cellulose decomposition and its associated microbial dynamics in soil were investigated in incubation experiments. Population dynamics of actinomycetes, bacteria, and fungi were monitored by direct counts. Populations of oligotrophic bacteria in cellulose-amended soil were determined by plate count using a low C medium containing 4 mg C liter⁻¹ agar, and copiotrophs using a high C medium. Cumulative ¹⁴CO₂ evolution from ¹⁴C-labeled cellulose was best described by a multiphasic curve in a 28-day incubation experiment. The initial phase of decomposition was attributed mainly to the activity of bacterial populations with a low oligotroph-to-copiotroph ratio, and the second phase mainly to fungal populations. An increase in oligotroph-to-copiotroph ratio coincided with the emergence of a rapid ¹⁴CO₂ evolution stage. Streptomycin reduced ¹⁴CO₂ evolution during the first phase and prompted earlier emergence of the second phase, compared to the control. Cycloheximide initially promoted ¹⁴CO₂ evolution but subsequently had a lasting negative effect on ¹⁴CO₂ evolution. Cycloheximide addition significantly increased bacterial biomass and resulted in substantially stronger oscillation of active bacterial populations, whereas it initially reduced, and then stimulated, active fungal biomass. The observed changes in ¹⁴CO₂ evolution could not be explained by observed shifts in fungal and bacterial biomass, probably because functional groups of fungi and bacteria could not be distinguished. However, it was suggested that oligotrophic bacteria prompted activation of cellulolytic enzumes in fungi and played an important role in leading to fungal dominance of cellulose decomposition.