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

the amount taken up at each site. The assessment methods will always provide
values which are optimum and comparable for different sites.
3. A number of plant factors such as the growth, withdrawal and reuse patterns for P
will determine the amount of P required for uptake. For a complete assessment of
P status of any specific site these factors, which describe the flux of P through the
system, should be included in the observation.
4. Forest soils are rich in organic matter and mostly highly acid. Any suggested
method should include these special characteristics of forest soils.
We have considered different alternatives and have come up with the following
preferred option of a suitable method to study medium to long term changes in P
status of forest soils in Germany.
Desorption of soil P by repeated sequential extractions using an anion based
appropriate extractant
This method is based on the common observation that the amount of P extracted by an
anion based mild extractant (or anion exchange resin) provides very low values for P,
which are not related to the dynamics of P in the soil. They may provide information
on short-term P changes in soils due to fertilization, litter changes, soil disturbance
etc. but fail to provide information on the long-term changes in soil P. Adequate
examples have been provided in the review to show that. This is primarily due to the
fact that single measurements of P pools fail to capture the dynamics of P desorption
from the soils which includes buffering capacity of labile P and the pattern of its
release. This can be achieved by repeating the extraction of a soil which is carried out
sequentially for sufficient number of steps so that the rate of P release starts to
decrease and sufficient data points are available to assess the amount of desorbable P
and the rate of desorption. An example of such a method is shown in Fig 14, where
two forest soils were sequentially extracted for a number of times using Bray 1
extractant (commonly used for acid soils) (Serrasolsas et al. in review).
As shown in Table 11 , the two soils were similar in many characteristics but
differed significantly in P values. Both soils were low in P retention capacities. P
fertilisation experiments on the two sites showed no growth response in one (Pinus
radiata plantation) but a big response in the other (Eucalyptus regrowth) stand. The
pattern of P released by stepwise extractions was fitted using the equation y = A (1-e-
Bx ) where y represented the cumulative amount desorbed for a cumulative x ml of
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