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

that these values should be validated for different soil types, but as a first
approximation a value of –25 can be used.
• By using the above values, Ulrich (1972) calculated the amount of mobilisable or
labile fractions of P for two types of sites: (a) for diluvial sands with medium
amounts of silicate content there may be 530 kg P /ha mobilisable P in 0-50cm
depth and (b) for an acid brown earth developed from loess mobilisable P content
in 0-25 cm depth may be about 1550 kg/ha.
• These values of mobilisable P are much higher than the small changes which are
expected to occur in the short period of 10 or 20 years of tree growth or even
during a period of one rotation. However P fractions may provide useful
information on substantial losses associated with erosion of surface soils or longterm
litter-raking activity where a significant proportion of organic or mobilisable
P is missing (Ulrich and Khanna 1969).
It is evident from the above discussion that it is possible to develop pedo-transfer
functions, but they cannot be developed based on the above given relationships for a
small number of sites. Depth-wise distribution of different P forms from a wide range
of soils will be required to test and develop such relationships further. In the context
of BZE analysis this does not seem to be a practical way to follow.
3.4 Suggested method to assess changes in P status of soils
There are a number of issues and requirements, which need careful consideration
before an appropriate method can be suggested for future use to assess changes in soil
P reflecting the P status of forest ecosystems. In our view the most important issues
are:
1. Short- versus medium- to long-term availability of soil P. It is important to
understand that contrary to the agricultural systems, where only short term
availability of P is of interest, forest ecosystems require both short- term and longterm
assessment of availability and the amount of P available. To determine shortterm
changes it is possible to adapt some of the methods developed in agriculture.
But for medium- to long-term changes forest-specific methods will be required.
2. The assessment of P availability will not always provide the amount that may be
taken up by the trees as there are site and plant specific factors which determine
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