Growth rate and response of bacterial communities to pH in limed and ash treated forest soils

Abstract

Culturable and total bacterial counts, bacterial growth rate and tolerance to pH, as well as microbial biomass, were studied in two coniferous forest soils. The pH had been changed by addition of lime and wood-ash from 4.3–4.4 to 7.0 in one soil and from 3.9–4.4 to 6.1 in the other. Higher total microbial activities and higher bacterial growth rates, measured as soil respiration rate and thymidine incorporation rate, respectively, were found in the treatments with increased pH. Similar effects of soil pH on the thymidine incorporation rate were found using two different methods, by measuring rates in either a soil slurry with all bacteria present or using a subsample of bacteria extracted from soil after homogenization-centrifugation. The number of culturable bacteria was up to 5.1 times higher in the high pH soils, while the acridine orange direct counts were unaffected by the treatments. Thus, the proportions of culturable bacteria increased in the limed and ash-treated soils compared to the untreated controls. An altered bacterial community composition due to liming was indicated by an altered response to pH, where the pH of the soil was correlated to the optimum pH for growth of the bacterial community. The ATP content of the soil was unaffected or increased in treatments with high pH compared to the control, while microbial biomass estimated by the substrate induced respiration technique (SIR) was always higher in limed and ash-treated plots.