A substrate-induced respiration (SIR) method for measurement of fungal and bacterial biomass on plant residues

Abstract

The substrate-induced respiration (SIR) method was modified and adapted to measure fungal, bacterial and total microbial contributions to glucose-induced respiration and the potentially active microbial biomass on decaying plant residues of differing composition. Decomposing residues from natural and agricultural ecosystems were chopped and sieved to include the > 1 mm fraction for routine SIR analyses on a continuous flow-through respiration system. SIR procedures were optimized for sample size (0.5–1.0g dry wt), glucose concentration (80mg g −1), antibiotic concentrations (16mg streptomycin g −1; 80 mg cycloheximide g −1), total solution volume (5 ml), antibiotic preincubation conditions (12 h at 4 C), and total assay time following glucose addition (2–3 h). Analyses of antibiotic selectivities for target populations were made from agar plate culture experiments with mixed residue-microbial populations under in vitro and in situ exposure to antibiotics. The results support those concentrations optimized by SIR and emphasize the importance of independent analysis of antibiotic selectivity. Measures of fungal, bacterial and total SIR (μg CO 2-C g −1 dry residue h −1) were linearly correlated ( P<0.001) with biovolume-derived estimates of total ( r 2 = 0.91) and FDA-active ( r 2 = 0.93) fungal biomass, bacterial biomass ( r 2 = 0.87) and total microbial biomass ( r 2 = 0.91), respectively. Fungal to bacterial SIR ratios (1.6:1 to 2.5:1) emphasize the dominance of fungi in the early stages of plant residue decay (⩽ 3 weeks). Plant residue SIR rates were 38–600 times greater than those of soils. Biomass specific SIR rates from plant residues (72 ng CO 2 -Ch −1 μg −1 biomass-C) were 5– 6 times higher than those reported from soil SIR. These findings suggest both a larger microbial biomass as well as a much higher proportion of physiologically active microorganisms on plant residues as compared to soils.