assessment of changes in the phosphorus status of forest ...

3. PHOSPHRUS IN FOREST SOILS
3.1 Introduction
Phosphorus occurs in many inorganic and organic forms in soils, but P is taken up by
roots from the soil solution phase in the phosphate forms. The amount of phosphate in
soil solution is commonly very low, which results from dynamic equilibrium between
dissolution and precipitation processes of inorganic P forms, mineralisation and
immobilisation of organic P forms and uptake by plant roots. Fig 11 describes in short
the conceptual frame work of P dynamics in soils. The P concentration in soil solution
determines the amount of P taken up by roots and mycorrhiza. It is described as the
intensity factor determining the uptake and other adsorption and desorption processes.
Low concentrations of P in the soil solution, which may be the result of kinetics of
desorption reactions, would reduce P uptake by roots. On the other hand, high P
concentrations in the solution phase, which may occur following P fertilization, would
lead to adsorption processes, initially in the readily labile forms which then move into
moderately or sparingly soluble forms. The moderately and sparingly soluble forms
are also in equilibrium with the occluded forms, though many of these reactions have
very slow kinetics. In addition, P bound in secondary and primary minerals may also
become available through weathering reactions mostly involving protons and
complexing agents produced by plants and microbes. Phosphorus taken up by the
plants is partly reverted to organic P forms, which may remain in labile forms or be
protected from further mineralisation. Mineralisation of organic P is carried out by
enzymatic reactions through phosphatases. Production of phosphatases is related to
root activities and the concentration of P in the solution or labile forms. Therefore,
plants play an important role in P dynamics of soils, especially in forest ecosystems.
In agricultural systems, where plants need high amounts of P over a short growing
period, contributions to the supply of P by mineralisation of organic P is commonly
considered to be of low significance. However, in surface layers of forest soils,
organic P may form the dominant fraction of P in most cases, but the amount and rate
of its mineralisation have been rarely studied. Harrison (1982) argued that owing to
the high amount of organic P in forest soils, mineralisation of a small fraction of about
1% of organic P in the surface 5 cm depth layer would supply sufficient P for annual
uptake. Bauhus and Khanna (1999) reported in their review about microbial biomass
in forest soils that microbial P can account for 13% to 23% of total P in the mineral
soil and forest floor layer respectively. Nutrients in microbial biomass are considered
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