Changes in bacterial and fungal biomass C, bacterial and fungal biovolume, ergosterol content, basal respiration and metabolic quotient (qCO2) after air-drying, rewetting and incubation (6 h and 2, 10 and 40 days) of four soil layers (L, F, H, ah) of an aspen (Populus tremuloides Michx.) forest and the F/H layer of a pine (Pinus contorta Loud.) forest in the Canadian Rocky Mountains were studied. Bacterial and fungal biomass were determined by selective inhibition of substrate-induced respiration (SIR) by streptomycin and cycloheximide. Bacterial and fungal biovolume were determined by epifluorescence microscopy using acridine orange and calcofluor white M2R for staining of bacteria and fungi, respectively. SIR inhibition was optimized by testing a wide range of inhibitor concentrations and different substrates.In general, air-drying (20°C, 14 days) caused only slight reductions (< 10%) in microbial biomass (SIR) in each of the layers. The effect of air-drying on bacteria and fungi depended on the soil materials. In general, most of the measurements indicated little changes in bacterial and fungal populations after air-dried layers had been rewetted. Basal respiration was increased strongly in each of the rewetted layers and C metabolized during the burst in respiration may have originated from killed microorganisms in aspen L, F and H, but other C resources presumably were also made available by air-drying of aspen ah and pine F/H.During incubation of rewetted layers bacterial biomass and biovolume increased for 10 days in most of the layers, resulting in a decrease in the fungal-to-bacterial ratio. Then, between days 10 and 40 fungi became more dominant in most of the layers and the fungal-to-bacterial ratio increased.Each of the indices measured to determine changes in bacterial and fungal populations were correlated significantly. The correlation between SIR inhibition by cycloheximide and ergosterol content was particularly high (r2 = 0.83) and an overall mean of 11 mg ergosterol g− fungal biomass C was obtained, indicating that changes in fungal populations in forest layers can be monitored accurately by measuring ergosterol contents. The usefulness of each of the methods used to characterize changes in bacterial and fungal populations in soil is discussed.