Introduction Directory UMM :Data Elmu:jurnal:A:Applied Soil Ecology:Vol15.Issue3.Nov2000:

Applied Soil Ecology 15 2000 273–281 Potential C-source utilization patterns of bacterial communities as influenced by clearing and land use in a vertic soil of Argentina E. Gomez a,∗ , V. Bisaro b , M. Conti c a Cátedra de Microbiolog´ıa Agr´ıcola, Facultad de Ciencias Agrarias, Universidad Nacional de Rosario, Campo Experimental J. Villarino, 2123 Zavalla, Argentina b Cátedra de Estad´ıstica, Facultad de Ciencias Agrarias, Universidad Nacional de Rosario, Campo Experimental J. Villarino, 2123 Zavalla, Argentina c Departamento de Suelos, Facultad de Agronom´ıa, Universidad de Buenos Aires, Av. San Mart´ın 4453, 1417 Buenos Aires, Argentina Received 20 April 2000; accepted 20 April 2000 Abstract 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 p0.001 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. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Agricultural intensification; Biolog assay; Soil microbial communities ∗ Corresponding author. Tel.: +54-341-456-7548; fax: +54-341-497-0199. E-mail address: egomezfcagr.unr.edu.ar E. Gomez.

1. Introduction

Interest in evaluation of soil quality has been in- creasing since the critical role of soil in determining the sustainability of ecosystems became clear. Soil quality has deep effects on system health and pro- 0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 7 8 - 0 274 E. Gomez et al. Applied Soil Ecology 15 2000 273–281 ductivity Doran and Parkin, 1996. Soil biological components are fundamental to the development of ecologically relevant process such as nutrient cycling, improvement of soil structure and xenobiotic decom- position. Microbial activity dominates the degradation of soil organic substrates and it is of major concern for ecosystem functioning. Microorganisms are largely sensitive to perturbations. Therefore, changes in the communities of soil organisms or in their functions may be early signs of alterations in soil health Dick et al., 1996; Toresani et al., 1998. Maintenance of biodiversity related to soil organisms seems to be cru- cial for agriculture to be considered sustainable Beare et al., 1995. Nevertheless, studies on diversity have been focused on higher organisms and only in the recent years more attention is being paid to microor- ganisms McLaughlin and Mineau, 1995; Kennedy and Gewin, 1997. Some researchers have reported variable responses of microbial communities to distur- bance. Lupwayi et al. 1998 found that conventional tillage decreased microbial diversity whereas reduced tillage enhanced it, while Hassink et al. 1991 did not find differences. Kennedy and Smith 1995 reported greater diversity indexes in cropped systems than in grasslands when they compared substrate use. The small size and morphological similarity make the complete morphological and taxonomic charac- terization of soil microbial communities impossible Garland and Mills, 1991. However, more than the analysis of taxonomic structure, biochemical and physiological properties may be suitable to a greater extent from an ecological point of view Hassink et al., 1991; Zak et al., 1994. In this respect, some authors have pointed out the need of throwing light on the effects of disturbances at a community level Kennedy and Smith, 1995. Garland and Mills 1991 applied to whole environ- mental samples a redox technique based in sole car- bon source utilization profiles, originally designed for strain identification, and found that this assay could be a useful tool for classifying bacterial communities. Color development due to the reduction of tetrazolium dye is used as an indicator of catabolism of each car- bon source. Zak et al. 1994 gave this method an ecological meaning, studying functional diversity by means of indexes of richness, diversity and evenness, and patterns that emerged from the catabolized carbon sources. Since then, and despite the successful use of car- bon substrate catabolism in terrestrial and aquatic ecosystems, recent literature refers to problems con- cerning the Biolog approach, specially as regards analysis and interpretation of results Insam, 1997; Insam and Hitzl, 1999. Thus, the Bilog system is considered more adequate for comparisons among microbial communities than for their characteriza- tion Glimm et al., 1997; Garland and Mills, 1999. Several findings have proved the assay to be useful to distinguish bacterial communities from differ- ent environmental and soil samples Lehman et al., 1997; Goodfriend, 1998. However, only a few stud- ies have dealt with the possibility of this method to show the effects of agricultural intensification on soil. In this work, we examined whether: a time since clearing and subsequent land use produced different sole carbon source patterns in the bacterial commu- nities of a vertic soil; or b two sampling depths showed distinctive substrate catabolism profiles. To as- sess whether relationships among bacterial communi- ties responded to years since clearing and subsequent management, patterns of potential substrate utilization were compared using the same soil, but in its native condition as reference, using the approach mentioned by Parkin et al. 1996.

2. Materials and methods