References

Influence of Microorganisms on Phosphorus Bioavailability in Soils 183
surrounding the roots (Watt et al. 1993; Gregory and Hinsinger 1999). Amellal
et al. (1999) found a significant aggregation and stabilization of rootadhering
soil by bacterial extracellular polysaccharides combined with an
increase in aggregate mean weight, diameter, aggregate macro-porosity,
adhering soil: root mass ratio, water-stable > 200 µm aggregates and 0.1–
2 µm elementary clayey microaggregates. Watt et al. (1993) reported a 10%
higher soil binding capacity of bacterial mucilages in comparison to plant
released materials.
Phospholipides in root and bacterial mucilages are powerful surfactants
that alter the interaction of soil solids with water and ions (Read et al.
2003). The occupation of adsorption sites as well as influences on aggregate
stability and water balance can facilitate phosphorus diffusion processes.
3.4
Release of Phosphorus from Organic Sources
Between 20 and 85% of the total P in agricultural soils are present in
the organic form, including inositol phosphate esters, phospholipids, nucleic
acids, phosphate linked to sugars and derivatives of phosphoric acid
(Tarafdar et al. 2001). Microorganisms mineralize organic materials like
plant residues and organic manures and enable the nutrient cycling. Using
an isotopic dilution technique, Oehl et al. (2001) found a daily mineralization
of 1.7 mg P kg −1 in an organically fertilized loamy silt soil. This amount
was approximately equivalent to soil solution P, indicating that mineralization
is a significant process in delivering available P. Although soil solutions
can include higher concentrations of organic than of inorganic phosphates,
plants can acquire phosphorus only in inorganic form (Tarafdar et al. 2002).
For this reason phosphatases which hydrolyse C–O–P ester bonds are very
importantforPnutrition.DependingontheirpHoptimum,acidandalkaline
phosphatases can be determined. Phosphodiesterase, which is able
to degrade nucleic acids, has not been extensively studied in soils (Dodor
and Ali Tabatabai 2003). Acid phosphatases are released by plant roots as
well as by microorganisms (Seeling and Jungk 1996; Yadaf and Tarafdar
2001), while alkaline phosphatases are probably mostly of microbial origin
(Tarafdar and Claasen 1988). The largest portion of extracellular soil phosphatases
is derived from the microbial population and strongly correlates
with microbial biomass (Dodor and Ali Tabatabai 2003). Tarafdar et al.
(2001) identified a better hydrolysis of lecithin and phytin by fungal acid
phosphatases in comparison to those of plant origin. Glycerophosphate
was equally hydrolyzed by both enzymes. Tarafdar et al. (2002) reported
that different fungi released only 25% of their acid phosphatases extracellularly,
but a 39 times higher extracellular phytase activity was noted in