Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2006
Publication Date: 5/1/2007
Citation: Caesar, T., Caesar, A.J., Gaskin, J.F., Sainju, U.M., Busscher, W.J. 2007. Taxonomic diversity of predominant culturable bacteria associated with microaggregates from two different agroecosystems and their ability to aggregate soil in vitro. Applied Soil Ecology. 36(1):10–21. Interpretive Summary: The secretion of extracellular polymeric substances (EPS) produced by bacteria, algae or fungi, and the adsorption of EPS to soil particles tend to aggregate the soil particles. These constitute the nuclei of stable microaggregates. Knowledge of the mechanisms by which microorganisms aggregate soil particles and the factors involved in such aggregation is needed, if soil microorganisms are to be manipulated to improve soil tilth. There is little work done to identify bacterial species in the microaggregates. We isolated and identified the culturable predominant aerobic bacteria from microaggregates of a field site cropped to barley (40 y, tillage) and a site cropped to alfalfa (9 y, no till). Artificial aggregates were generated by mixing individual bacteria species with natural soil then they were tested for their water stability and their strength in the laboratory. Results indicated that some bacterial species aggregate soil better than others and that the species that were the most efficient soil aggregators were isolated from microaggregates of non disturbed soil (alfalfa). This study is an initial step in providing methodologies for the survey in microaggregates of the aerobic bacterial communities that can be cultured in laboratory, and for investigating their function to aggregate soil.
Technical Abstract: Soil macroaggregates (>250 µm) are easily disrupted when wetted quickly, whereas microaggregates (50- to 250 µm) are highly stable. There is little knowledge of functional groups of bacteria from microaggregates that can stabilize soil and their diversity. We isolated the predominant bacteria from microaggregates collected from two agroecosystems: a site cropped to barley (40 years with tillage) and to alfalfa (9 years with no tillage) and assayed for their potential to aggregate soil in an in vitro assay. Soil suspensions from physically disrupted microaggregates were spiral-plated and bacterial colonies were isolated from the most dilute portion of the spiral then purified. To characterize the culturable bacteria thus isolated, fatty acid methyl ester (FAME) profiles, substrate utilization patterns (Biolog), and DNA sequence analysis using 16s rRNA amplification region were performed for bacterial identification. Principal component analysis of FAME profiles and Biolog patterns was conducted to describe patterns of community composition of bacterial species within each agroecosystem. Gram-negative bacteria in microaggregates from barley soil consisted entirely of Pseudomonas spp. whereas Stenotrophomonas, Sphingobacterium, Chryseobacterium, and Acinetobacter spp. were found in addition to the Pseudomonas in microaggregates of alfalfa soil. Artificial aggregates were generated by amending soil with pure cultures of various species identified from microaggregates of both soils and tested for their water stability and their strength. The results indicated that Stenotrophomonas and Sphingobacterium spp. exhibited the greatest ability to stabilize soil and to increase aggregate strength among several species tested. This study demonstrated that microaggregates harbor functional groups of bacterial species capable of aggregating soil particles in vitro, and there were a greater range of such species in microaggregates of non disturbed soil (alfalfa) than in disturbed, yearly tilled soil (barley). Our results provide a baseline for future studies of the community structure and dynamics of the predominant aerobic, culturable bacteria from microaggregates found in diverse habitats with a capacity to aggregate soil.