Agriculture Reference
In-Depth Information
Table 9.1
Production of organic acids by PSB grown in broth culture
PSB
strains
Oxalic acid
(mg l
1
)
Malic acid
(mg l
1
)
Succinic acid
(mg l
1
)
Propionic acid
(mg l
1
)
PSB1
0.052
0.042
0.150
0.031
PSB6
0.020
0.043
0.125
0.019
PSB9
0.025
0.070
0.245
0.011
PSB10
0.010
0.062
0.175
0.009
PSB14
0.008
0.025
0.205
0.008
PSB15
0.008
0.026
0.172
0.020
PSB16
0.011
0.050
0.250
0.028
Modified from Panhwar et al. (
2012
)
PSB
phosphate-solubilizing bacteria
filtrates, which revealed that the insoluble P might be solubilized by mechanisms
other than process of acidification. Soil inorganic P is mostly solubilized by
production of organic acids. Low-molecular-weight organic acids play multiple
roles in the soil processes, such as root nutrient acquisition, mineral weathering,
microbial chemotaxis, and metal detoxification (Jones et al.
2003
). Phosphate-
solubilizing bacteria can release numerous organic acids including oxalic, citric,
butyric, malonic, lactic, succinic, malic, gluconic, acetic, glyconic, fumaric, adipic,
and 2-ketogluconic acid (Leyval and Berthelin
1989
). Among the organic acids,
oxalic and malic acid amounts are more common than the others (Zeng et al.
2008
).
Phosphate-solubilizing bacterial strains belonging to the genera
Pseudomonas
,
Bacillus
, and
Rhizobium
are among the most powerful P solubilizers (Sharma
et al.
2013
; Rodriguez et al.
1999
). Some of the organic acids released by PSB
strains are listed in Table
9.1
.
9.3.2 Phosphate Solubilization by Phosphate-Solubilizing
Bacteria
9.3.2.1 Solubilization of Al and Fe-bound Soil P
The solubilization of Fe and Al takes place through the release of protons by PSB,
reducing the adsorbing surface charge to make possible the sorption of negatively
charged P ions. Phosphate sorption also might be decreased with the release of protons
by acidification that increases H
2
PO
4
in comparison to HPO
4
2
having higher
similarity to reactive soil surfaces (Whitelaw
2000
). The different forms of P, like
Al-P and Fe-P, are mostly solubilized by carboxylic acids (Khan et al.
2007
;Henri
et al.
2008
) by the mineral P dissolution as an effect of anion exchange of PO
4
3
or by
chelation of Al and Fe ions associated with P (Omar
1998
). This is due to high affinity
of iron uptake system by root-colonizing pseudomonas which depends on the release
of Fe
3+
-chelating molecules like siderophores (Altomare et al.
1999
). Furthermore, P
is replaced by carboxylic anions through ligand exchange from sorption complexes
