Plants may be an important selective force for the diversity of rhizosphere populations of bacteria and fungi through their influence on soil nutrients. Rhizo-sphere soil can be defined as that volume of soil adjacent to and influenced by plant roots (Metting, 1993). It is a region of intense microbial activity because of its proximity to plant root exudates, making rhizosphere microbial communities distinct from those of bulk soil (Curl and Truelove, 1986; Whipps and Lynch, 1986). Micro-bial activity is stimulated in this area because of the nutrients provided by the root or germinating seed (Rouatt and Katznelson, 1961). Microbial populations and functioning in an agroecosystem are influenced by the root and the soil environment, including mineral and organic material. The aboveground plant community can influence microbial spatial heterogeneity in soil. Those microorganisms that respond to root exudates or related substrates will dominate the rhizosphere. Decaying root systems also function as a source of nutrients for the surrounding microorganisms (Swinnen et al., 1995). Microbial populations decrease with distance from the roots (Yeates and Darrah, 1991).

Bacteria account for the largest number of inhabitants of the rhizosphere. Gramnegative, non-spore-forming rods with simple nutritional requirements are stimulated more by roots than are coccoid forms and Gram-positive, spore-forming rods (Curl and Truelove, 1986). The composition of the plant community may influence the diversity of the microbial community due to the variability in chemical composition of the exudates (Christensen, 1989).

The development of rhizosphere microbial communities also is influenced by different plant species (Rovira, 1956), plant phenology (Smith, 1969), and environmental factors influencing plant growth (Rovira, 1959; Vancura, 1967; Martin and Kemp, 1980). Bacterial and fungal abundance in the rhizosphere is influenced by the nutrient status of both plant and soil. The percent mycorrhizal cover on roots of Plantago lanceolata was positively correlated with leaf nitrogen and phosphorus, while the percent cover of bacteria and other fungi was negatively correlated with phosphorus (Newman et al., 1981). Nitrogen fertilization increased numbers of fungi and Gramnegative bacteria in a rice rhizosphere (Emmimath and Rangaswami, 1971). It may be difficult to separate the effects of soil nutrients on rhizosphere populations from effects involved with increased or altered root exudation of organic compounds. Grasses grown in monoculture can modify nitrogen availability (Wedin and Tilman, 1990), and it has been hypothesized that such changes in soil nitrogen availability influenced by plant species affects the composition of vesicular arbuscular mycor-rhizae fungal communities (Johnson et al., 1992b).

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