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These methods provide interesting information on the rates of transfers of nutrients and carbon within the ecosystem, but the details of the transfers (sources and sinks) and transfer rates (source and sink strengths) are currently a major focus of research.

In addition to interactions among ectomycorrhizal fungi and saprotrophic fungi in soil, there has been shown to be close associations of these mycorrhizal fungi and bacteria within the rhizosphere. In his reviews, Garbaye (1991; 1994) discusses that importance of these so-called helper bacteria (Garbaye and Bowen, 1987; 1989; Duponnois and Garbaye, 1990; 1991; Fitter and Garbaye, 1994). It is suggested that these bacteria assist in the fungal/plant recognition system (Duponnis, 1992), receptivity of the host root to the mycorrhizal fungus and enhancement of the mycorrhizal fungal mycelial growth (Duponnois and Garbaye, 1990; Garbaye and Duponnois, 1992) prior to root colonization, alteration of the rhizospheric soil by altering pH, production of ion-complexing compounds (siderphores) and the nutrient balance (Wallander and Nylund, 1991), and germination of fungal propagules (Ali and Jackson, 1989), which is probably most important in arbuscular mycorrhizae (von Alten et al., 1993). The main ecological implication of these helper bacteria, however, is to enhance the rate of root colonization, assist in the acquisition of nutrients by the production of enzymes, and to complex ions that help to detoxify soil for plant growth. In addition, there are synergistic interactions among mycorrhizae and other bacteria in soil, particularly root nodulating and free-living, nitrogen-fixing bacteria (Barea et al., 1997). These associations are particularly important in agroecosystems, in which the sustainability of soil fertility is of prime importance. The actual variation in the species composition of bacteria around ectomycorrhizal roots in the field, however, is largely unknown. New methods for the identification of these communities (Mogge et al., 2000), together with more information about the physiological attributes of these bacteria, such as phosphate solubilization (Berthelin and Leyval, 1982; Leyval and Berthelin, 1983; Singh and Kapoor, 1998), will allow us a greater understanding of the ecological and ecosystem processes that accrue from these close associations among mycorrhizae and bacteria.

Although there is a large body of information showing that different species of trees associate with different communities of ectomycorrhizae, there is less information on the host plant/arbuscular mycorrhizal species specificity. It is generally thought that there is low specificity; however, Eom et al. (2000) showed that after 4 months of growth there were significantly different arbuscular mycorrhizal communities developing under different component plant species from a tallgrass prairie (Fig. 3.25).

Barni and Siniscalco (2000) investigated the changes in arbuscular mycorrhizal colonization of roots of plants in a vegetation succession following agricultural disturbance. They compared the mycorrhizal associations of u

FIGURE 3.25 Mean spore densities of arbuscular mycorrhizal fungi Glomus interradices, G. mosseae and Glomus spp. indicating differences in the mycorrhizal community structure on roots of five plant species (Solidago missouriensis, Poa pratense, Panicum virginiatum, Sporobolus heterolepis and Baptisia bracteata). Data from Eom et al. (2000).

FIGURE 3.25 Mean spore densities of arbuscular mycorrhizal fungi Glomus interradices, G. mosseae and Glomus spp. indicating differences in the mycorrhizal community structure on roots of five plant species (Solidago missouriensis, Poa pratense, Panicum virginiatum, Sporobolus heterolepis and Baptisia bracteata). Data from Eom et al. (2000).

the component plant species in an agricultural field with fields abandoned for up to 3 years, supporting ruderal vegetation, a grassland, shrubland, and early successional and late successional woodlands. They showed that shortly after agricultural field abandonment, the ruderal vegetation was composed primarily of annual species, which were largely nonmycorrhizal. After 2 to 3 years more perennials were recruited into the community and most plant species had arbuscular mycorrhizal symbionts as droughting became more of a problem and available nutrients in soil were reduced. It was only at the later stages of woodland establishment that the arbuscular mycorrhizae were displaced by ectomycorrhizae, where both a change in plant host species and accumulation of organic plant residues in the woodland phase, led to a dominance of ectomycorrhizae, which have the ability to utilize organic forms of nutrients.

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