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Title: SOIL AND PLANT EFFECTS ON MICROBIAL COMMUNITY STRUCTURE

Author
item Buyer, Jeffrey
item Roberts, Daniel
item RUSSEK-COHEN, ESTELLE - UNIV OF MARYLAND

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2002
Publication Date: 11/26/2002
Citation: BUYER, J.S., ROBERTS, D.P., RUSSEK-COHEN, E. SOIL AND PLANT EFFECTS ON MICROBIAL COMMUNITY STRUCTURE. CANADIAN JOURNAL OF MICROBIOLOGY. 48:955-964. 2002.

Interpretive Summary: The rhizosphere is the narrow zone of soil directly adjacent to plant roots, containing root exudates, leaked and secreted chemicals, sloughed root cells, and mucilages. This complex mixture of organic compounds provides nutrients for microorganisms. Many studies show much higher numbers of bacteria and fungi in the rhizosphere than in the bulk soil. This is referred to as the 'rhizosphere effect'. However, only a small percentage of soil bacteria can be grown in the laboratory, and many species of bacteria are not detected by isolation plating, so many of the papers on the rhizosphere effect which relied on isolation plating may not be valid. We hypothesized that the rhizosphere effect would be strongest for fast growing microorganisms that favor higher concentrations of nutrients because they could respond very quickly to the nutrient levels in the rhizosphere. We used 3 different methods to look at microbial community structure of the rhizosphere and soil. The total microbial community, studied by analysis of lipids from soil and rhizosphere samples, was strongly affected by soil type but not by plant species, and there was very little rhizosphere effect. An assay for bacteria that grew rapidly on different carbon sources did show a strong rhizosphere effect, but corn and soybean rhizosphere communities were very similar to each other. There was little rhizosphere effect on the fungal community. These results were consistent with our hypothesis and indicated that the rhizosphere effect may be limited to a very small percentage of the soil bacteria. Microorganisms are a critical component of soil, crucial to soil quality and sustainability, and a better understanding of soil and rhizosphere microbial ecology is needed to improve soil resource management.

Technical Abstract: We investigated the effects of two different plant species (corn and soybean) and three different soil types on microbial community structure in the rhizosphere. Our working hypothesis was that the rhizosphere effect would be strongest on fast-growing aerobic heterotrophs, while there would be little or no rhizosphere effect on oligotrophic and other slow-growing microorganisms. Communities were characterized by soil fatty acid analysis and by substrate utilization assays for bacteria and fungi. Fatty acid analysis revealed a very strong soil effect but little plant effect on the microbial community, indicating that most soil microorganisms were not affected by deposition of nutrients from roots. There was a strong rhizosphere effect detected by the substrate utilization assay for bacterial communities, with soil controls and rhizosphere samples clearly distinguished from each other. However, corn and soybean rhizosphere samples had very similar microbial communities by this method, indicating that the specific plant species had little effect. There was a much weaker rhizosphere effect on fungal communities than bacterial communities as measured by the substrate utilization assays. At this coarse level of community analysis the rhizosphere microbial community was impacted most by soil effects, and the deposition of nutrients by roots only affected a small fraction of the total bacteria.