Agriculture Reference
In-Depth Information
7.1
Introduction
Phosphorus (P) is a major growth-limiting nutrient, but unlike nitrogen, there is no
large atmospheric source of P that can be made biologically available (Ezawa
et al.
2002
). Root development, stalk and stem strength, flower and seed formation,
crop maturity and production, N
2
fixation in legumes, crop quality, and resistance to
plant diseases are some of the important attributes associated with P nutrition. Soil
P dynamics is characterized by physicochemical (sorption-desorption) and biolog-
ical (immobilization-mineralization) processes. Large amount of P applied as
fertilizer enters in to the immobile pools through precipitation reaction with highly
reactive Al
3+
and Fe
3+
in acidic soils and Ca
2+
in calcareous or normal soils
(Gyaneshwar et al.
2002
; Hao et al.
2002
). Efficiency of P fertilizer throughout
the world is around 10-25 % (Isherword
1998
), and concentration of bioavailable P
in soil is very low reaching the level of 1 mg kg
1
soil (Goldstein
1994
). Soil
microorganisms playing a key role in soil P dynamics and subsequent availability of
P to plants (Richardson
2001
; Khan et al.
2007
) are quite often referred to as
phosphate-solubilizing microorganisms (PSMs).
Inorganic forms of soil P are solubilized by PSMs through organic acids
production that dissolve P minerals and/or chelate cationic partners of the P ions,
i.e., PO4
3
directly, releasing P into solution (He et al.
2002
). Phosphate-
solubilizing bacteria (PSB) among PSM have been used as biofertilizer since
1950s (Kudashev
1956
; Krasilinikov
1957
). Release of P by PSB from insoluble
and fixed/adsorbed P is, therefore, an import aspect of P availability in soils. There
are strong evidences that soil bacteria can transform soil P to the forms available to
plant. Microbial biomass on the contrary assimilates soluble P and prevents it from
adsorption or fixation (Khan and Joergensen
2009
). Microbial community also
influences soil fertility through other soil processes, for example, decomposition,
mineralization, and storage/release of nutrients. Even though microbial inoculants
are in use for improving soil fertility since long, research on P solubilization has
inadequately been done compared to N
2
fixation. Considering the gap in this area,
the ecological perspectives, diversity, mechanism of P solubilization, and role of
PSB in plant growth promotion are highlighted here in this chapter. Furthermore,
various biotechnological tools currently employed to better understand the plant
rhizosphere and its associated microbiota are discussed.
7.2 Distribution, Diversity, and Rhizosphere Competence
of Phosphate Solubilizers
Evidence of naturally occurring rhizospheric PSM dates back to 1903 (Khan
et al.
2007
). Among PSM, fungi more effectively solubilize P than bacteria
(Alam et al.
2002
). On the contrary, of the whole soil microbial populations, PSB
constitute 1-50 %, while PS fungi (PSF) accounts for only 0.1-0.5 % (Chen
