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

assess P status or any changes in P status of soils by using simple methods, rather the
complex nature of interactions requires special techniques often involving a number
of different methods. It was shown that P in the soil solution phase determines P
uptake but it is the most dynamic component of soil P. Through solubility product
diagrams it has been possible to assign the dominant inorganic forms of P which may
be determining the concentrations of P in soil solutions. A conceptual framework is
normally used to assign differences in the lability of different soil P fractions with
respect to their availability for uptake. Sequential extraction methods are used to
obtain P fractions which are differently labile. However, not the easily labile fractions
undergo changes in the long term but in many cases the fractions of moderate to low
turnover indicating the dynamic equilibria occurring among different forms of P. This
creates a problem for selecting the suitable chemical fractions which could be
extracted by appropriate methods. Consequently there are many different types of soil
P tests which are used both in agriculture and forestry. Agricultural methods have
been developed specifically to provide fertilizer recommendations for crops, and are
thus of limited use to describe the long-term P status of forest soils. The limitations
and advantages of various methods have been described.
We recommend a method including repeated extractions of soil P by using an
appropriate extractant. The selection of extractant(s) would require some initial
studies. Such a repeated desorption of P from soils will provide a capacity factor
giving the amount of labile fraction and a desorption rate function which may be
related to soil characteristics. Any change in those two parameters would then
indicate a change in P status of soil. The development of the method should be done
on a small but representative set of soils of the BZE collective. In order to save
resources, for the rest of the samples these parameters may be obtained by developing
suitable models using NIRS or MIRS techniques. The proposed method will still
require a high level of resource commitment to be made available in future under the
BZE program. Despite the major concerns of changes in P status of forest soils, this
area of research has not received sufficient attention in the past.
4. ACKNOWLEDGEMENTS
A number of persons provided ideas, data evaluation and support for this document.
We wish to thank for their efforts: Dr. Carl Hoecke, Dr. Jan Evers, Dr. Henning
Meesenburg and Dr. Michael Mindrup.
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