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

Mineralisation of organic P does not follow the same pattern as N, whereas very little
N is released during the initial phases of litter decomposition, causing the C:N ratio to
increase in the decomposing litter, a significant amount of P is released, before it
follows the same pattern of decomposition as for N.
3.2.4 P uptake - Role of rhizosphere and mycorrhiza activities
Presence of mycorrhiza may facilitate the uptake of P under acidified rhizosphere
conditions. Gillespie and Pope (1991) reported that P plant uptake rates were similar
at both pH 4.0 and pH 7.0 by non-mycorrhizal black locust (Robinia pseudoacacia
L.), whereas in plants with mycorrhizal associations, the rate of phosphorus uptake at
pH 4.0 was twice that observed at pH 7.0. Depending on the soil type five-fold
differences in the P diffusion rates were observed in the six forest soils using
desorption-isotherm buffer power assays. There are many studies in this special area
of research and the Mycorrhizal Information Exchange (MIE) website at the
following address provides the pertinent literature
(http://mycorrhiza.ag.utk.edu/).
The following issues should be mentioned in the context of this review:
1. Phosphate uptake is a diffusion processes which occurs a few mm around the fine
roots and mycorrhizal filaments, thus the rhizosphere play an important role in the
amount of P taken up by plants.
2. A number of rhizosphere processes would affect the solubility of inorganic P, the
most important ones are the changes in pH, exudation of inorganic acids by roots.
3. Mycorrhiza play an active role in the uptake of P by intensive exploring of soil
volume, and through exudation of acids and enzymes.
4. Phosphate being an anion, other anions will compete with its uptake. Thus
interactions with other ions in the rhizosphere are important determining factors
for P uptake.
5. Fine roots and mycorrhiza are highly sensitive to acid conditions and high levels
of nutrients in the soil.
3.2.5 Use of soil tests – limitations and possibilities
Many chemical solutions are used to extract P from soils. With any extracting
solution, four basic reactions are involved by which P is removed from the solid
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