Potential C-source utilization patterns of bacterial communities as influenced by clearing and land use in a vertic soil of Argentina

A sole-carbon-source catabolism assay (Biolog GN microplate) was used to study whether bacterial communities from the same vertic soil, but under different management history, showed distinctive patterns of C-substrate utilization. Two sampling depths (0-7.5 and 7.5-15 cm) were also investigated. The response of microbial communities to increasing periods of time - 16, 26 and 40 years (S2, S3, S4, respectively)- since native vegetation clearing and to land use was evaluated as related to the soil in its native condition (S1). Tenfold dilutions of soil suspensions were performed and aliquots of 10(-4) dilution were inoculated into each well of the Biolog GN microplates and then incubated. Activity on C-substrates was recorded as optical density at regular time intervals. Absorbance data from the 54-h incubation time were used to calculate the average well-color development (AWCD) in each plate, richness (number of catabolized C-sources) and diversity Shannon's index. Principal component analysis (PCA) was performed to study patterns of C-source utilization. Number of bacteria was determined by plate counts on to tryptic soy agar (TSA) and expressed as colony-forming units (CFU) g(-1) soil. The lowest AWCD values were found in the 40 years since clearing site (S4) in both depths, despite the fact that the largest number of bacteria was found in the top 0-7.5 cm. Samples from the native condition showed the largest richness and diversity on metabolized C substrates p <0.001) while S4 had the lowest values at a depth of 0-7.5 cm. The locations that were investigated could be differentiated by PCA. The Biolog GN assay showed to be sensitive to distinguish soil bacterial communities from sites with different times elapsed since clearing and management history. Larger differences among samples were detected at 0-7.5 cm depth. Distinctive patterns of 'in vitro' C-source utilization could be related to differences in chemical composition of soil organic matter. (C) 2000 Elsevier Science B.V. All rights reserved.