References

Interactions Between Mycorrhizal Fungi and Bacteria 203
phosphate-enriched and 32 P-labeled soils. These studies found that dual
inoculation induced a lowering in the SA of the host plants indicating that
these used extra 31 P solubilized from other not directly available P sources,
either endogenous or added as rock phosphate.
A model experiment (Barea et al. 2002c) is summarized here to describe
the phosphate-solubilizing bacteria x mycorrhiza interactions on P capture,
cycling and supply. This experiment involved a factorial combination
of four microbial and two chemical treatments. The microbial treatments
were: (1) mycorrhiza inoculation; (2) phosphate-solubilizing rhizobacteria
inoculation; (3) mycorrhiza plus bacteria dual inoculation; and (4) noninoculated
controls, exposed to the naturally existing mycorrhizal fungi
and phosphate-solubilizing bacteria. The two chemical treatments were:
(1) nonamended control without P application, and (2) rock phosphate
application. Labeling was done by mixing the soil thoroughly with a solution
containing 32 P phosphate ions. Plant seedlings were transplanted
after soil labeling. The 32 P activity in the plant material was measured,
and the specific activity of P was calculated by considering the radioactivity
per amount of total P content in the plant (Zapata and Axmann 1995).
Both rock phosphate addition and microbial inoculation improved biomass
production and P accumulation in the test plants, with dual microbial inoculation
as the most effective treatment. Independently of rock phosphate
addition, mycorrhiza-inoculated plants showed a lower specific activity
( 32 P/ 31 P) than their comparable nonmycorrhizal controls. If the 32 P/ 31 P
ratio in soil solution is uniform both spatially and temporally, similar
SAintheplantsshouldbeassumedwhetherornottheyaremycorrhizainoculated.
However, in the reported experiments, the SA values were lower
in mycorrhiza-inoculated plants than in the corresponding controls, particularly
when they were inoculated with phosphate-solubilizing bacteria.
This means that mycorrhiza-inoculated plants were taking soil P which
is labeled differentially from that taken up by nonmycorrhiza-inoculated
controls. The explanation could be that phosphate-solubilizing bacteria,
either inoculated or naturally present, were effective in releasing 31 Pfrom
sparingly soluble sources, either from the soil components or from the
added rock phosphate. This release of P ions would constitute a part of the
total 31 P pool from which the mycorrhizal mycelium taps phosphate and
transfers it to the plants. Such microbial activities could result in the lower
SA in dually inoculated plants. It can, therefore, be concluded that the interactions
between mycorrhizal fungi and phosphate-solubilizing bacteria
display a fundamental role for P-cycling, a fact of considerable interest in
ecosystems. These conclusions are graphically illustrated in Fig. 2.