References

Interactions Between Mycorrhizal Fungi and Bacteria 197
phate (and other nutrients) from the soil solution, in such a way that the
external mycorrhizal mycelium is responsible for a major part of the P
acquired by the plant (Joner et al. 2000). Because of the low mobility of
phosphate ions, a phosphate-depletion zone develops around plant roots
(Jeffries and Barea 2001), thus, the mycorrhizal activity, due to the ability
of external hyphae to exploit soil volumes that are not accessed by nonmycorrhizal
roots, is a critical issue for nutrient capture and cycling in
soil–plant systems (Smith and Read 1997). In cooperation with other soil
organisms, the external mycorrhizal mycelium forms water-stable aggregates
necessary for good soil tilth (Miller and Jastrow 2000). Mycorrhizal
fungi also improve plant health through increased protection against biotic
and abiotic stresses (Jeffries et al. 2003).
Accordingly, some interactions between these beneficial microorganisms
are particularly relevant to benefit plant fitness and soil quality (Barea et al.
2002a, b) and merit further considerations. The aim of this chapter is to analyze
some key examples of these interactions involving fungi and bacteria
which benefit two important properties of soil quality, i.e., nutrient mobilization
and cycling and soil structure stabilization. Because almost all plant
species growing in terrestrial ecosystems are mycorrhizal (Smith and Read
1997), mycorrhizal fungi will have a central position in the interactions to be
studied. The mycorrhizosphere (Barea et al. 2002a, b) interactions related
to nutrient cycling concern three types of soil bacteria: (1) plant symbiotic
N2-fixing rhizobial bacteria; (2) phosphate-solubilizing bacteria; and
(3) phytostimulators Azospirillum. Representative experiments using isotope
( 15 Nand 32 P) dilution techniques to ascertain the extent of soil–plant
benefitfromtheseinteractionswillbedescribed.Theactivityofmycorrhizal
fungi with regard to soil aggregation, known to occur in interaction
with saprophytic microorganisms, including bacteria, will also be reviewed
as these are fundamental for soil structure stabilization.
3
Interactions Between Mycorrhizal Fungi
and Symbiotic N2-Fixing Rhizobial Bacteria
Nitrogen fixation is a key factor in biological productivity, it being accepted
that more than 60% of the N-input to the plant community has a biological
origin, and that half of this input is due to the symbiotic plant–bacteria
systems, particularly those involving legumes (Postgate 1998). The bacterial
partners in the symbiotic relationships with legume species belong to
the genera Rhizobium, SinoRhizobium, BradyRhizobium, MesoRhizobium,
and AzoRhizobium, collectivelytermedasRhizobium or rhizobia. Their
association with legume roots leads to formation of N2-fixing nodules