(1) The document discusses how root-soil interactions influence the bioavailability of phosphorus through changes in root morphology, rhizosphere pH, and release of root exudates. (2) Root exudates like organic acids, amino acids, and enzymes released into the rhizosphere can solubilize inorganic phosphorus by chelating cations binding phosphate ions or ligand exchange. Microorganisms in the rhizosphere also mediate the availability of inorganic and organic phosphorus. (3) Phosphorus deficiency leads to changes in root morphology like increased root/shoot ratio and formation of cluster roots to enhance phosphorus acquisition from the soil. Root exudates and rhizosphere microbes

1. Bioavailability of soil and applied phosphorus
as influenced by root-soil interaction

2. Contents
Introduction
Bioavailability of phosphorus
Change in root morphology and architecture
Change in pH of rhizosphere
Release of root exudates
Microbial medieted availabiity of inorganic and
organic phosphorus
Bioavailability of applied phosphorus

3. Introduction
Rhizosphere:
The concept of bioavailability was first given by Hiltner in 1904. It is the soil
surrounding a root, in which physical, chemical and biological properties have
been changed by root growth and activity both in radial and longitudinal
direction along an individual root (Brimccombe et al., 2007).
Bioavailability:
It is the ability of the soil–plant system to supply essential plant nutrients to a
target plant, or plant association, during a specific period of time as a result of
the processes controlling -
(1) the release of nutrients from their solid phase in the soil to their solution
phase;
(2) the movement of nutrients through the soil solution to the plant root–
mycorrhizae; and
(3) the absorption of nutrients by the plant root–mycorrhizal system
(Comerford, 1998).
It is the process of supplying nutrients to biological organisms.
Soil Nutrient Bioavailability: A Mechanistic Approach
(By Stanley A. Barber)

4. Regulation of root architecture in plant under
P-deficient condition
Lynch and Brown, 1998

5. Effect of P deficiency on root morphology
Wang et al., 2008
14 DAT
21 DAT
35 DAT
Phosphorus deficiency increases root/shoot.
Phosphorus was supplied at 0, 1, 10, 25, or 75 μM (P0, P1, P10, P25 or P75) as KH2PO4.
Low P supply increase
no. of lateral roots.
Low P supply increased the
number of the lateral roots but
decreased elongation of these
lateral roots compared with
adequate P supply, leading to the
formation of cluster-like
Roots.
High P supply 25µM and
75µM,higher shoot/root.

6. Root clusters are positioned in soil horizons or patches that
have above-average total P but low levels of plant available P
(Pate et al., 2001; Lambers et al., 2011).
There are dual uptake system with high affinity transporters and
low affinity transporters. However, it is the adaptive significance
of differential expression of high-affinity transporters i.e. it
adjust it activity according to soil P supply (Lambers et al., 2006).
Process involved in uptake of phosphorus are mainly mass flow
and diffusion, which contribute 5 and 95%, respectively as
estimated from field grown maize (Barber, 1995).
This explains that the bioavailability of soil P is likely to be much
larger for plants which can efficiently take up P at low P
concentrations (Hinsinger, 2001).

7. Role of phytohormones in root morphology
Phytohormone Organism (Phytostimulators) Role of phytohormone
Auxin Azospirillum
Enterobacter
Pantoea
Pseudomonas
Regulation of lateral root initiation
Root hair position
Gravitropism
Cytokinin Arthrobacter
Azospirillum
Pseudomonas
Paenibacillus
Promote root hair development
inhibiting primary and lateral root
elongation
Gibberelin Azospirillum
Azotobacter
Bacillus
Herbaspirillum
Gluconobacter
Root elongation
Lateral root extention
Ethelene Azospirillum Root hair development
Richardson et al., 2009

8. P level
(kg P ha–1)
P. bilaii
innoculation
Root length
(m/plant)
Specific root length
(m/g root DW)
0 With 2.35 67
Without 3.48 80.8
6.4 With 2.43 74.8
Without 2.45 72.5
19.3 With 2.23 70.5
Without 2.45 78.2
Effect of microbe (Penicillium bilaii ) on root morphology
Vessey et al., 2000

9. Effect of pH on form and availability of P
In acid soils trivalent Fe and Al can occur in large concentrations
in the soil solution, whereas they will be negligible at neutral or
alkaline pH (Lindsay, 1979). Conversely, in neutral and alkaline
soils, Ca and, to a lesser extent Mg will be the dominant cations
in soil solution

10. Mechanisms involved in change in pH of rhizosphere
Production of CO2 and hence H2CO3 in rhizsohere by root
and microbial respiration (Fageria et al., 2011).
Root ion exchange (Gahoonia et al., 1992).
Nitrogen fixation by rhozobium-legume symbiosis (Fageria et
al., 2006).
Change in redox potential of rhizosphere (Fageria et al.,
2011).
Organic matter decomposition (Fageria et al., 2011).
Deficiency of Phosphorus (Akhtar et al., 2006) and Iron in
soil (Marschner, 1995).

13. Root exudates released by plants to rhizosphere
Felix et al., 2002

14. Function of root exudates
Component Rhizosphere function
Phenolics nutrient source, chemoattractant signals to microbes,
microbial growth promoters
chelators of poorly soluble mineral nutrient
Organic acids nutrient source, chemoattractant signals to microbes,
chelators of poorly soluble mineral nutrients
Amino acids and
phytosiderophores
nutrient source, chelators of poorly soluble mineral
nutrients, chemoattractant signals to microbes
Vitamins promoters of plant and microbial growth
nutrient source
Purines nutrient source
Enzymes catalysts for P release from organic molecules
biocatalysts for organic matter transformation in soil
Root border cells stimulate microbial growth
release chemoattractants
Hawes et al., 1998; Dakora and Phillips, 1996

15.  Among root exudates, citric and malic acids were the most
frequently involved in such a response of plant roots to P
starvation (Pearse et al., 2006).
 Exudation of organic acids/anions, released in responses of
either Al toxicity and P deficiency or Fe deficiency (Richardson
et al., 2011).
 Effectiveness of individual organic acid to mobilize phosphorus
depend upon no. of carboxylic group they posses and follow the series
Tricarboxylic > Dicrboxylic> Monocarboxylic acid
(citrate) (Malate)
Venkalas et al., 2003; Khademi et al., 2009
General nature of organic anions

16. Plant species Organic anion
released
(Dominant)
References
White lupin, Alfalfa Citric acid Lipton et al., 1987;
Watt and Evans, 1999
Maize, Wheat, Oilseed
rape, Tomato
Malic acid Neumann and
Römheld, 1999
Sugar beet Oxalic acid Gerke et al., 2000
Organic anions released by different plants

17. Mechanism of phosphorus bioavailability by root exudates
Felix et al., 2002
chelation
Legand
exchange+compexation

18. Release of organic acid in
response to phosphorus
starvation
Solution P concentrations (x10-6 molL-1) as a function of organic acids (micro mole mL-1 of wheat
.
Negative correlation
positive correlation
Negative correlation
Datta et al., 2003

19. Differential release of root exudates released by legume and
nonlegume crop
Plant Genotype Root exudation
(14C released,
dpm/
mg root dry wt.)
Free amino
acids
(mg leucine
equiv./
g root dry wt.)
Reducing
sugars
(mg glucose
equiv./g root
dry wt.)
Wheat Ganga-11 106.26±7.21 0.49±0.053 3.50±0.75
Deccan-103 65.11±5.14 0.37±0.012 2.70±0.39
Mean 84.19±6.17 0.43±0.032 3.20±0.57
Green
gram
PS-16 174.50±11.2 0.16±0.03 1.70±0.27
Pusa-105 321.17±17.0 0.21±0.02 2.30±0.19
Mean 247.84±14.1 0.19±0.025 2.00±0.23
Singh and Pandey, 2003

20. Organic acid mediated phosphorus extraction
Khademi et al., 2009

21. Ca5(PO4)3OH + 7H3O+ ↔3H2PO4
− + 5Ca2+ + 8H2O
(hydroxyapatite)
Mechanisms involving dissolution hydroxyapatite
The equilibrium can be shifted to the right, i.e., the dissolution of
the hydroxyapatite can be enhanced if-
a. protons are supplied.
b. Adsorption of P ions or plant uptake.
c. Complexation/ligand exchange of Ca
by an organic ligand such as citrate
or oxalate.

22. Root exudates and rhizosphere effect
Rhizosphere is characterised by significant increase in number
and activity of soil microorganism due to exudation of
photosynthetic carbon from roots.
Microorganisms Rhizosphere soil Bulk soil R/S ratio
Bacteria 1.2x109 5.3 x107 23
Actinomycetes 4.6x107 7.0 x106 7
Fungi 1.2 x106 1.0 x105 12
Protozoa 2.4 x103 1.0 x103 2
Algae 5.0 x103 2.7 x104 0,2
Ammonifiers 5.0 x108 4.0 x106 125
Denitrifiers 1.26 x108 1.0 x105 1260
Vega et al., 2007

23. Phosphorus solubilizing mocroorganisms
P solubilizers References
Bradyrhizobium, Rhizobium Antoun et al. (1998)
Gordonia Hoberg et al. (2005)
Enterobacter Kim et al. (1997b)
Rahella Kim et al. (1997a)
Panthoea Deubel et al. (2000)
Pseudomonas Deubel et al. (2000); Hoberg et al. (2005)
Aspergillus, Penicillium,
Trichoderma
Barthakur (1978)
*Pseudomonas Richardson and Hadobas (1997)
Aspergillus, Emmericella,
Penicillum
Yadav and Tarafdar (2003)
Telephora, Suillus
(ectomycorrhizal fungi)
Colpaert et al. (1997)
*Phytase producer

24. Bacteria are more effective in phosphorus solubilization than
fungi (Alam et al., 2002) .
Among the whole microbial population in soil, PSB constitute 1 to
50%, while phosphorus solubilizing fungi (PSF) are only 0.1 to
0.5% in P solubilization potential (Chen et al., 2006).
Strains from bacterial genera Pseudomonas, Bacillus, Rhizobium
and Enterobacter along with Penicillium and Aspergillus fungi are
the most powerful P solubilizers (Whitelaw, 2000).
Phosphorus solubilizing ability of PSM

25.  A recent study by Esberg et al. (2010) showed correlation
between microbial respiration and changes in NaOH
extractable P and suggested that microbial access to this
fraction was greater.
 Mechanisms for inorganic P solubilization by PSRB, are (i)
release of organic and inorganic acids, and (ii) excretion of
protons that accompanies to the NH4 + assimilation (Vega et
al., 2007).
 The hydroxyl and carboxyl groups of acids inorganic acids
chelate cations (Al, Fe, Ca) and decrease the pH in basic soils
(Kpomblekou and Tabatabai, 1994; Stevenson, 2005).
Solubilization of in inorganic phosphorus

26.  Phosphorus desorption potential of different carboxylic anions are
in the order: citrate > oxalate > malonate / malate > tartrate >lactate
> gluconate > acetate > formiate (Ryan et al., 2001).
Mechanism involving solubilization of inorganic phosphorus
Neutral or alkaline soils
(Dicalcium phosphate) CaHPO4 + H+ H2PO4
- + Ca2+
(Hydroxyapatite) Ca5(PO4) 3(OH) + 4H+ 3HPO4
2- + 5Ca2+ + H2O
Acid soils
(Strengite) FePO4.2H2O HPO4
-2 + chelate-{Fe3+}+ OH- + H2O
(Variscite) AlPO4.2H2O HPO4
-2 + chelate-{Al3+}+ OH- + H2O

27. 0 2 4 6 8 10 12 14
Effect of root exudates on solubilization of inorganic P
Chen et al., 2002

28. Solubilization of organic P
Chen et al., 2002

29. Mineralization of organic phosphorus
Plants and microorganisms increase exudation of P-hydrolysing
enzymes under P deficiency. Phytase specifically catalyses the
break-down of phytate, the major form of organic P in soils
(Rengel and Marschner, 2005).
Microbial enzymes show higher efficiency for P release (Tarafdar et
al., 2001). Roots excrete little, if any, phytase, whereas
microorganisms (eg. Aspergillus niger) exude large amounts
(Richardson et al., 2001) .
The combination of citrate and enzymes in the root exudate severy
effective in hydrolyzing organically bound soil phosphorus when
the compounds in soil humic substances are chemically bonded to
metals, especially aluminum and iron..

30. Lamber et al., 2006
Mechanism of humic substance solubilization

31. Accumulation and turnover of microbial biomass P
Microbial P in bulk soil is 2% to 10% of total soil P, and within soil surface this
may be as much as 50% (Oberson and Joner, 2005).
Achat et al. (2010) has reported a faster cycling of a major component of the
soil microbial P pool, or fast pool (accounting for 80% of the total microbial P),
with a turnover time of less than 10 d in an organic-P-dominated forest soil.
Immobilization of P within the biomass is an important mechanism for
regulating the supply of P in soil solution (Seeling and Zasoski, 1993).

32. Micorrhiza mediated bio availability of phosphorus
In case of micorrhiza, the calculated inflow of phosphate through
external hypae is approximately 1000-fold faster than phosphate
diffusion rate through soil.
Bieleski, 1973

33. Sinergistic effect of micorrhiza and PSRB in P bioavailability
 The mycorrhizal hypha also release
carbonaceous compounds into the
surrounding soil forming a niche called
“mycorrhizosphere” (Rambelli, 1973,
Linderman, 1988).
BacteriaAdvantages:
Higher release of
carbonaceos substances in
micorrizosphere.
Efficient uptake of
phosphate released by
PSRB (less refixation).
As long as PSRB remain in
micorrhizosphere carbon
requirement is fulfilled.

34. Sinergistic effect between Enerobacter. agglomerans (PSRB) and
Glomus etunicatum (AM)
Kim et al., 1998
Treatments Fresh
weight/plant(g)
Dry
weight/plant(g)
P conc. In dry
tissue (%)
Total P/plant(g)
Control 352.34 42.21 0.28 116.46
PSRB 385.91 48.49 0.26 125.26
AM 404.26 47.62 0.25 120.94
PSRB+AM 455.01 54.56 0.25 134.41

35. Solubilization of phosphate rock
Mechanism of rock phosphate solubilization
H+ excretion Organic acid
production
Acidification of
rhizosphere
Chelation
Legand
exchange
(Both for plant and microbes)
Melissa et al., 2006

36. Dissolution of rock phosphate by rhizosphere acidification
Species pH of
soil
Treatment pH in Water Olsen-P
Rhizosphere Non-rhizosphere
Brachiaria 4.9 Control 5.47 5.18 —
MFPR 7.39 7.46 −0.72
RPR 5.47 5.53 7.20
NCPR 5.83 5.67 5.32
TSP 5.28 5.40 22.40
Stylosanthes 4.9 Control 4.74 4.82 —
MFPR 7.34 7.30 −1.09
RPR 4.44 5.22 13.63
NCPR 4.76 5.29 12.24
TSP 3.99 4.87 25.53
Maria et al., 2007

37. Puptakemg/pot
Green manure FYM Control
Effect of organic P sources on enzyme activity and P uptake by rice
Bhadraray et al., 2002

38. Summary

39. Conclusions
Bioavailability is the process of supplying nutrients to biological
organisms.
Bioavalability of P to plants in rhizosphere are change by various
processes that are induced either directly by activity of plant roots
or by activity of rhizosphere microflora.
Rhizosphere pH changes by various mechanisms and this has
profound effect on bioavailability of P.
Root exudates released by plant roots directly take part in
enhancing bioavailability of P by mechanism of ligand exchange/
chelation or indirectly do it by increasing no. of micro-organisms in
rhizosphere.
In P deficient condition plant increases it’s surface foraging activity
by changing it’s root morphology and architecture .
Rock phsphate applied to soil solubilized by both plant microbes
by photon (H+) excretion or by excretion of organic acids.

40. Some drawbacks of the concepts
Desorption of sorbed P mostly occure via ligand exchange
mechanism . Numerous work have demonstrated that metal
oxides and clay sorbent surfaces have stonger affinity of P ion
than other competing organic or inorganic ligands . Affinity of
kaolinite surfaces for anions decreased in the following order:
phosphate > citrate > bicarbonate
Mechanism pH change and P bioavailability don’t work well in
soil having more than 2% CaCO3 and high buffering capacity.
Except some extreme cases such as white lupin and oil seed
rape , which exihibit top range of exudation rate , other plant
species exude very less amount so it is questionable.
Organic acid anion and enzyme phosphateses sometimes either
become adsorbed in soil matrix or decomposed by microbes.

41. Future thrust area
There are different processes occuring inside rhizosphere for
increasing bioavailability of P . Soreaerh is needed to examine
the relative contribution of various processes for increasing
bioavailability P in rhizosphere .
Abillity of various plants and among plants varios species
and genotypes vary in all above processes , which are less
documented . So in future more and more genotypes shoude
be identified and expoited in breeding programme.
Ecological considerations of different group of micro-
organisms and interact in rhizosphere to mobilize P different
fractions should be studied properly.
More and more compatible combinations like PSRB and AM
should be identified in future .