CHARACTERIZATION OF A SUBSTRATE-INDUCED RESPIRATION METHOD FOR MEASURING FUNGAL, BACTERIAL AND TOTAL MICROBIAL BIOMASS ON PLANT RESIDUES

A substrate-induced respiration (SIR) method is described to measure the contributions of fungi and bacteria to total glucose-induced microbial respiration on plant residues of differing composition. Relationships between fungal, bacterial and total SIR and biomass were used to develop regression equations for predicting microbial biomass C from measures of SIR. Total SIR rates (100-2000-mu-g CO2-C g-1 h-1) and biomass-specific SIR rates (64 - 72 ng CO2-C h-1 mu-g-1 biomass C) from plant residues were considerably greater than those calculated from the literature for soils. Results of longer term decomposition studies indicate that the C:N ratios of plant residues through time account for the greatest amount of the variation in total SIR. Annual decomposition rate constants (k) for plant residues were positively correlated (r2 = 0.99) to overall mean estimates of total SIR. The plant residue SIR method has advantages over conventional direct count methods because it distinguishes a physiologically active component of the microbial biomass. Furthermore, it allows separation of fungal and bacterial components that may aid in understanding microbial controls on plant residue decomposition.