Agriculture Reference
In-Depth Information
carboxylic acids synthesis, but proton release cannot be the single mechanism
(Deubel et al.
2005
).
4.4.1 Solubilization of Iron Phosphate/Aluminium
Phosphate
Solubilization of Fe and Al by PSMs occurs via proton release accompanied by
decrease in the negative charge of adsorbing surfaces to facilitate the sorption of
negatively charged P ions. Proton release can also decrease P sorption upon
acidification which increases H
2
PO
4
in relation to HPO
4
2
having higher affinity
to reactive soil surfaces (Whitelaw
2000
). Carboxylic acids mainly solubilize Al-P
and Fe-P (Khan et al.
2007
; Henri et al.
2008
) through direct dissolution of mineral
P as a result of anion exchange of PO
4
3
by acid anion or by chelation of both Fe
and Al ions associated with phosphate (Omar
1998
). Root-colonizing
Pseudomo-
nads
with high-affinity iron uptake system based on the release of Fe
3+
-chelating
molecules, i.e. siderophores (Altomare et al.
1999
), have been reported to solubilize
bound Fe. Moreover, carboxylic anions replace P from sorption complexes by
ligand exchange and chelate both Fe and Al ions associated with P, releasing P
available for plant uptake after transformation. Ability of organic acids to chelate
metal cations is greatly influenced by its molecular structure, particularly by the
number of carboxyl and hydroxyl groups. Type and position of the ligand in
addition to acid strength determine its effectiveness in the solubilization process
(Kpomblekou and Tabatabai
1994
).
4.4.2 Mineralization of Organic Phosphate
Mineralization of soil organic P plays an imperative role in P cycling of a farming
system. Organic P may constitute 4-90 % of the total soil P. Approximately half of
the soil and rhizosphere microorganisms possess P mineralization potential which
is catalysed by enzymes, for instance, phosphatases (Tarafdar and Claassen
1988
).
Phosphatase enzymes are present in all organisms but only bacteria, fungi and some
algae are able to secrete them outside of their cells. There are two different kinds of
phosphatases such as acid phosphatases and alkaline phosphatases which use
organic P as a substrate to convert it into inorganic form (Beech et al.
2001
).
Principal mechanism for mineralization of soil organic P is indeed the production
of acid phosphatases (Hilda and Fraga
1999
). Acid phosphatase by plant roots/
microbes (Yadav and Tarafdar
2001
) or alkaline phosphatase (Tarafdar and
Claassen
1988
) enzymes hydrolyse the soil organic P or split P from organic
residues. Soil organisms such as
Bacillus and Streptomyces
sp. have been reported
to mineralize very complex organic P by producing extracellular enzymes and
