Agriculture Reference
In-Depth Information
in greenhouse and in field trials and have been found to efficiently promote plant
growth in P-deficient soils (Bashan et al.
2013
). Concurrently, numerous commer-
cial PSB-based biofertilizers have been developed and are being marketed on a
global scale. However, these biofertilizers are not used as commonly as N-fixing
bacteria (NFB) in crop production systems, and even in many cases their effective-
ness in the soil-plant system has been reported quite uncertain, particularly under
field conditions. Still, they remain a very attractive alternative to chemical phos-
phatic fertilizers, particularly in poor regions of the world.
5.5.1 Mechanisms of Inorganic-P Solubilization: A Brief
Account
It is widely agreed that the most important mechanism evolved in bacteria to
solubilize sparingly soluble forms of P
i
in the soil is related to the abundant
production and excretion of organic acids (Rodriguez and Fraga
1999
; Goldstein
2007
; Khan et al.
2010
). This is mainly accomplished by bacterial transformation of
sugars exuded through the roots into their respective sugar acids, whose amount and
nature varies with the type of available sugars (Deubel et al.
2000
). Even though
several organic acids including citric, glutamic, succinic, lactic, oxalic, glyoxalic,
maleic, fumaric, tartaric, and a-ketobutyric acids have been shown to efficiently
mobilize P
i
from soils (Khan et al.
2006
), two of them are particularly effective:
gluconic and 2-ketogluconic acids, probably due to their extremely low pKa
(s) (~3.4 and ~2.6, respectively) (Goldstein
1995
; Rodriguez and Fraga
1999
).
Gluconic and 2-ketogluconic acids are produced through direct oxidation
(or non-phosphorylating oxidation) of glucose, an alternative aldose-utilization
pathway which is expressed in a number of rhizobacteria (Goldstein
2007
).
Nevertheless, other mechanisms have also been proposed to explain PO
4
2
release
from P-containing minerals, in the absence of a noticeable acidification of the
extracellular milieu. The
sink theory
considers that continuous microbial uptake
of P from water solution would disturb the chemical equilibrium between soluble
and insoluble forms of P, favoring thus the solubilization process (Halvorson
et al.
1990
). The
acidification by H
+
excretion theory
, proposed by Illmer and
Schinner in
1995
, claims that ammonium ion assimilation by bacteria would release
protons that will act, in turn, as agents for P solubilization. This would occur in the
absence of any organic acid production. The
bacterial surface adsorption theory
is based on the fact that bacterial interaction with P-containing minerals, through
exopolymeric substances, will cause both an increase in the extent of mineral
dissolution and an inhibition of secondary mineral formation (Wightman and Fein
2004
; Rong et al.
2008
). The
chelating theory
proposes that the chelating property
of the organic acid anions is as important as the proton effect. In such a context, any
effective biological chelator—siderophores, for example—would enhance both
mineral dissolution and P solubilization (Campbell and Eick
2002
; Hamdali
