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

sequential extraction scheme of extracting inorganic fractions, assign a degree of
lability to each of the organic fraction (Tiessen and Moir 1993). Table 8 shows one
such sequential fractionation method for agricultural soils, which has also been widely
used for research purposes for forest soils. We shall discuss the use of these methods
for assessing changes in the forest soils of Germany.
Sequential extraction methods are highly resource consuming. As can be seen in
Table 8, 7 different extractions and analyses are required to quantify the different P
fractions. Therefore these methods are commonly reserved for research purposes, but
are probably not suitable for routine monitoring purposes, where large numbers of
samples are involved (e.g., BZE). However, there are potentially two alternatives
available:
(a) to develop pedo-transfer functions suitable for calculating these different fractions
in soils from soil parameters that are simple to analyse (e.g. Total P or C in the
soil) (Ulrich and Khanna 1968, Ulrich 1972), and
(b) to develop a purpose-based analytical method such as to determine available P,
where specific chemical extraction procedures are employed. A typical example
of this approach is the wide use methods for quantifying available soil P in
agriculture. These soil testing methods are specifically designed for
recommending suitable doses of fertilizer inputs to crops. The most commonly
used methods are outlined in Table 9. These methods include simple extraction
with water (water extracts from soils which have accumulated high amounts of P
through fertilizer inputs in easily dissolved forms), mild exchange of P present on
solid surfaces (anion resin methods), mild extractants using anions (lactate
methods Egner and Riehm, bicarbonate method by Olsen et al; fluoride methods –
Bray; acid methods – Mehlich; and many others), methods described by Kuo
(1996). To predict fertilizer demand of annual crops in agricultural soils, the
amount of P obtained by chemical extractants is related to plant P uptake, which is
measured through biomass growth and P concentration in plant tissue. Through
suitable regression and other statistical methods, critical soil test values are then
derived. Evidently, the purpose and usefulness of such methods for forest soils are
highly questionable. Firstly, the annual P uptake is extremely difficult, if not
impossible to quantify, since new tree tissue can also be built with P that has been
stored within the tree or mycorrhiza, or has been retranslocated from ageing or
senescing tissues. Thus the system is much more buffered against short-term
fluctuations in P supply from soil. Secondly, owing to the fact that P is commonly
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