References

206 J.M. Barea, R. Azcón and C. Azcón-Aguilar
conclusionsfromthisreviewaresummarizedbelow,beforethereportof
a model experiment carried out in this laboratory (Requena et al. 2001).
According to Miller and Jastrow (2000), understanding the contributions
of mycorrhizal fungi to the formation and stabilization of soil aggregates is
necessary to realize the hierarchical nature of the mechanisms involved in
aggregation. In the frame of this chapter, only the microbially mediated processes
will be mentioned (see Miller and Jastrow 2000 for information on
other factors). In a first stage, soil particles are bound together by bacterial
products and by structures of saprophytic and mycorrhizal fungi into stable
microaggregates (2–20 µm in diameter). These are bound by microbial
products into larger microaggregates (20–250 µm in diameter), in which
bacterial polysaccharides act as binding agents. Microaggregates are then
bound into macroaggregates (> 250 µm in diameter), a process in which
bacterial polysaccharides act as binding agents and the mycorrhizal mycelia
contribute by increasing the size of macroaggregates. Such a mycorrhizal
role is accounted for by the size, branching habits and three-dimensional
structure of the external mycelium colonizing the soil surrounding the root
(Miller and Jastrow 2000), an activity that can persists up to 22 weeks after
theplanthaddied(TisdallandOades1980).
The effect of the mycorrhizal mycelium in the formation of water-stable
soil aggregates has been evidenced in different ecological situations (Andrade
et al. 1995, 1998; Bethlenfalvay et al. 1999; Requena et al. 2001), and the
involvement of glomalin, a glycoprotein produced by the external hyphae of
mycorrhizal fungi, has been demonstrated (Wright and Upadhyaya 1998).
Glomalin has been suggested to contribute to hydrophobicity of soil particles
and, because of its glue-like hydrophobic nature, it also participates in
the initiation and stabilization of soil aggregates (Miller and Jastrow 2000).
The importance of mycorrhizosphere interactions in improving soil
structure was investigated in a revegetation experiment aimed at restoring
a degraded Mediterranean ecosystem in southern Spain (Requena et al.
2001). This experiment used a shrub legume belonging to the natural succession
and dual mycorrhizal and rhizobial inoculation, a biotechnology
that has received considerable attention in the last decade (Herrera et al.
1993). The experiments carried out by Requena et al. (2001) aimed at assessing
the long-term benefits of inoculation with these two types of plant
microsymbionts not only on the establishment of the target legume species,
but also on changes in key physicochemical soil properties known to affect
soil quality (Kennedy and Smith 1995). In particular, the effect on soil structure,
plant nutrient availability, organic matter content, microbial activity,
etc., was analyzed as their degradation is often concomitant to disturbance
of natural plant communities, as a result of the degradation/desertification
processes(JeffriesandBarea2001).
The experiments were carried out under field conditions in a represen-