Agriculture Reference
In-Depth Information
biofertilization purposes have been claimed. For example, fungal hyphae are able to
traverse longer distances in the rhizosphere and bulk soil and to firmly attach to
P-containing particles (Kucey
1983
; Chabot et al.
1993
). Also, fungi do not lose the
P-dissolving activity upon repeated subculturing under laboratory conditions.
Moreover, some fungal species are able to mobilize P
i
from sparingly soluble
minerals, like iron oxides (Delvasto et al.
2007
).
Under certain circumstances, the combined inoculation of more than one species
may benefit plants better than either group of organisms alone. Dual-inoculation
assays have clearly shown that field effects of PSB may be enhanced upon their
mixed inoculation with P-solubilizing fungi (PSF) (Gull et al.
2004
; Khan
et al.
2006
). Not surprisingly then, combined inoculation of PSF and PSB resulted
in enhanced growth, nutrient uptake, and yield in several crops. For example, dual
inoculation of wheat with an arbuscular mycorrhizal fungus (AMF),
Glomus
etunicatum
, and PSB augmented all monitored plant growth and yield parameters,
in experiments conducted in pots containing P-deficient soil (Saxena et al.
2014
;
Minaxi et al.
2013
). These are among the main reasons explaining why mixed
populations of soil bacteria and fungi are currently prepared and sold as commercial
biofertilizers for improving P nutrition of plants (Richardson
2007
).
In the context of mountain agriculture, however, only a few studies have been
published concerning the potential use of CT-PSF as biofertilizers. The first report
concerning the isolation, characterization, and identification of CT-PSF was
published in 2008 (Pandey et al.
2008
). From soil samples collected in the IHR, a
total of 246 fungal isolates, representing 36 genera and 72 species, were isolated.
After a thoroughly screening procedure, eight species of PS
Penicillium
were
finally selected for further investigation. These isolates solubilized P in vitro after
15-21 days at 21
C, and this ability was correlated with acidification of the culture
medium. They also produced acid and alkaline phosphatases. Additionally, some
isolates showed a wide range of tolerance for temperature, pH, and salt concentra-
tion. Very similar results were also obtained by this research group when studying
ten
Aspergillus
species, isolated from the same IHR soil samples (Rinu and Pandey
2010
). Among the species tested,
A. niger
exhibited the highest P-solubilizing
activity at low temperatures (9 and 14
C), after 5-6 weeks of incubation in vitro.
This preliminary study demonstrated the potential of CT
Aspergillus
species to be
developed as “bioinoculants” for application in cold mountainous regions. The
ability of
A. niger
to solubilize and release P
i
was further confirmed by Singh
et al. (
2011
). This time, two strains of
A. niger
were isolated not from mountainous
ecosystems but from Spitsbergen, the largest island of the Svalbard Archipelago in
the Arctic region. Both isolates showed maximum PS activities at pH 7.2 and 20
C
in bioassays conducted in vitro, but they were not studied further. To confirm their
potential as good candidates for developing biofertilizers to be used in acidic soils,
containing sparingly soluble Al and Fe phosphates, three CT
Aspergillus
species,
namely,
A. niger
,
A. glaucus,
and
A. sydowii
, were tested in vitro in the presence of
different carbon sources (Rinu et al.
2013
). Even though all the three species
mobilized P from the P-containing minerals tested,
A. niger
gave the best results:
it solubilized 32 % and 8 % of the supplemented Al and Fe phosphate, respectively.
