Fungi are well known for having metabolic capabilities that allow them to decompose complex organic material. Additionally, most fungi are found predominately in the organic horizon where complex organic compounds are abundant. The mineral horizon has a greater proportion of bacteria. These trends suggest that fungi are the predominate decomposers of complex polymeric substrates while bacteria focus on simpler substrates. However multiple strains of cellulose degrading bacteria have been identified making the relative contribution of these groups to decomposition less clear. The goal of this research was to quantify the assimilation of cellulose and glucose derived carbon by bacterial and fungal taxa thus revealing their decomposition activity. To address this objective we examined bacterial and fungal communities from soils with differing carbon substrate inputs by sampling the Detritus Input and Removal Treatment (DIRT) plots in Harvard Forest. Specifically, we examined soils that have received twice the normal amount of leaf litter (double litter), had no leaf litter (no litter), or had normal litter inputs (control). We measured the decomposition of glucose and cellulose by incubating soils with 13C-labeled substrates. Assimilation of substrates by microbial taxa was assessed by qSIP (quantitative stable isotope probing). This method measures the incorporation of isotopically labeled (i.e. 13C) compounds into the DNA of microorganisms.
The rate of glucose decomposition was greater than that of cellulose regardless of treatment, with the majority of mineralization occurring in the first two weeks. Decomposition of glucose proceeded most rapidly in the control (normal litter input) soil and slowest in the no litter soils. In contrast cellulose was most extensive in the double litter treatment. Both fungal and bacterial taxa showed significant 13C incorporation of glucose and cellulose. There was a greater amount of 13C substrate incorporated into the DNA of fungi from the cellulose in the double litter treatment than the control and no litter samples. Bacteria rapidly assimilated 13C from glucose in the control and double litter treatments while uptake was lower in the no litter soils. Fungi were the primary consumers of the cellulose in the double litter treatment while bacteria dominated in the no litter treatment. These results indicate that the relative contribution of bacterial and fungal taxa to simple and complex organic matter decomposition is dependent upon the historical resource availability within the ecosystem.