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

3.3 Use of pedo-transfer functions to assess availability and the amount of
available soil P
Pedo-transfer functions (PTF) are predictive functions for soil properties, which are
not easy to measure but can be derived from other more-available, easily, routinely or
cheaply measured soil parameters. McBratney et al. (2002) used the term ‘soil
inference system’, where pedo-transfer functions are the knowledge rules for soil
inference engines. A soil inference system takes measurements with a given level of
certainty (source) and by means of logically linked pedo-transfer functions (organiser)
infers unknown data with minimal inaccuracy (predictor). Pedo-transfer functions
thus utilize different regression analysis and data mining techniques to extract rules
for soil properties which are difficult to measure.
For this review we intend to consider the possibility of using pedo-transfer functions
to assess the amount of different P forms in forest soils, which would enable the
evaluation of long term soil P changes in forest ecosystems. Earlier attempts have led
to regression relationships which were based on 12 different sites collected in Lower
Saxony (Ulrich and Khanna 1972, Khanna and Ulrich 1969, Ulrich 1972). The basis
of these relationships are described below in short:
• Total P (Pt) is considered to be a function of organic C in the surface soils which
can be described by the equation Pt = a+b (%C). Here the assumption is that Pt
follows the depth-wise distribution parallel to that of C and the parameter ‘a’
describes that fraction of P in soil, which does not undergo any biologically driven
P transformation and translocations. The value of ‘a’ is thus based on the type of
bedrock material, which was found to be 5 mg P /100g for silicate-poor diluvial
sand (site - Meppen), 10 mg P/g for the diluvial sand which had medium amounts
of silicates (site - Sprakensehl), 32 mg P/g for the soil from loess on sandstone
(site – Solling). The value of ‘b’ in the equation can range from 2 to 7 depending
upon the soil type and rooting depth. The significance of assessing the distribution
of total P in soil in relation to organic matter content is indicated by the role of
biological parameters in the transformation and translocation of P in a soil profile.
• Now assuming that occluded P fractions (Poccl) (which also includes secondary or
primary silicates) are not taken up by the roots and are not involved in the
retranslocation processes, one can define the fraction of occluded P as : Poccl / Pt x
100 = a+b log(%C). The values of parameter ‘a’ may vary from 30-45% and that
of parameter ‘b’ which has negative values of 18 – 31. Ulrich (1972) suggested
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