Factors controlling phosphorous availability and their relevance for phosphorous nutrition of forest stands

During the development of forest soils, phosphorus (P) commonly declines in both abundance and availability and thus becomes limiting for biological activity. Forest ecosystems adapt to this process by developing efficient P recycling mechanisms, including the formation of stable P-stores and optimized P uptake by trees. In Central Europe, this long-term soil evolution has been disturbed by an accelerated soil acidification driven by anthropogenic deposition. The fast decrease in P nutrition observed in various forests supports this hypothesis. 

Work package A: Relative contribution of soil compartments to P- uptake and uptake/mobilization efficiency and...

Work package B: Characterization of soil solid phase P – pools and shifts between them

These work packages are worked on by Stefan Holzmann in the frame of the project of Dr. PD von Wilpert and Prof. Dr. Bauhus at the Forstliche Versuchs- und Forschungsanstalt Freiburg (FVA).

Work package C: The ability of trees to shape their rhizosphere in order to cope with low P availability

The objective of this workpackage is to assess to which extent trees contribute actively to P mobilisation. In the comprehensive definition within this project, this includes acidification or alkalinisation of the (mycor)rhizosphere, exudation of carboxylates and phosphatases by root segments and hyphae, and affecting the activity of rhizosphere microorganisms including their exudation of potentially P mobilising compounds. In order to address these questions, cross-exchange experiments in rhizoboxes are carried out in the greenhouse. Tree seedlings from a given provenance are grown both in soil from the same provenance and in soil from a provenance differing in P availability and speciation. By comparing the plant internal P status with P mobilisation and the occurrence of potentially P mobilising substances in the rhizosphere, it will be tested to what extent the plant provenance determines its reaction to different soil environments in terms of root exudation. We also study the effects of the root exudation on the activity and community structure of the rhizosphere microorganisms. Furthermore, using a 33P labelling approach, the competition between the plant and rhizosphere microorganisms for mobilized P is assessed. Linking the results from all three parts, should enable us to assess to which degree a trees P nutrition depends on the plants own ability to exude P mobilising substances into the rhizosphere, on its ability to stimulate and possibly form a symbiosis with specific microorganisms, or on it just passively taking advantage of P mobilised by microorganismes. Finally, the results of these experiments will help to explain the effect of planted tree seedlings on P mobilisation and transport in the percolation experiments with undisturbed soil cores in workpackage A.