Unraveling trees’ P sources based on the linkage of bottom up und top down approaches for P uptake modelling

The contribution of organic and mineral sources to the P uptake of trees is still unresolved. The hypothesis that organic sources get more important for the P nutrition of trees with decreasing P stocks of soils is in the centre of our project. Our approach of testing this is the integrative use of bottom-up and top-down approaches for the  modelling of the P uptake by trees. The joint SPP project provides unique opportunities to follow this approach. Phosphorus   requirements of old-grown beech forests will be assessed performing top-down modelling based on growth data and litter production. The bottom-up model will provide the potential uptake of the root system based on the absorbing root surface and the potential supply from mineral and organic sources. The assessment of the specific root length will be combined with the measurement of extramatrical hyphae by (micro)morphological methods using reflecting microscopy. The analyses of P-mobilization kinetics will be performed using diffusive gradients in thin films (DGT) at low soil temperatures (only inorganic P) and warm soil temperature (inorganic and organic P). The data will then be fitted to a kinetic process model. Specific effects by exudation of carboxylates and synergisms of root networks, and the effect of soil moisture will be assessed by microdialysis. Variability of P availability that can be related to soil structure will be assessed by the fractionation of soil aggregates. The different working packages of
our project will provide relevant knowledge on the processes  controlling P uptake at the study sites. In addition our approach will link soil and plant analyses and will help synthesize manifold data related to P mobilization and plant uptake gained from the study sites. We do not expect a consistent P-budget of the contrasting forest sites, but the experiments and the combination of the different model  approaches will serve as a framework for the classification of input variables regarding their sensitivity to the responding P uptake.