The rhizosphere of mycorrhizal plants

Providing that appropriate carbon substrates are available, microbial communities are able to develop a range of activities which are crucial in maintaining a biological balance in soil (Bowen and Rovira 1999), a key issue for the sustainability of either natural ecosystems or agroecosystems (Kennedy and Smith 1995). Soil-borne microbes have a particular microhabitat in which to flourish. In particular, they are bound to the surface of soil particles or found in soil aggregates, while others interact specifically with the plant root system (Glick 1995). The root-soil interface is actually a dynamic changing environment, a microcosm where microorganisms, plant roots and soil constituents interact (Lynch 1990; Azcon-Aguilar and B area 1992; Linderman 1992; B area 1997, 2000, Kennedy 1998; Bowen and Rovira 1999; Gryndler 2000), to develop what is known as the rhizosphere (Hiltner 1904). The rhizosphere, therefore, is the zone of influence of plant roots on the associated microbiota and soil components, and is clearly a different physical, chemical and biological environment from the bulk soil (Bowen and Rovira 1999), where an altered microbial diversity and increased activity and number of microorganisms is characteristic (Kennedy 1998).Actually, the structure and diversity of populations of fluorescent pseudomonads associated with roots were shown to differ significantly from those of soil populations. Rhizosphere and non-rhizosphere populations could be discriminated on the basis of their ability to use specific organic compounds (Lemanceau et al. 1995; Latour et al. 1996), to mobilize ferric iron (Lemanceau et al. 1988) or to reduce nitrogen oxides (Clays-Josserand et al. 1995).