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

We analyse how plant-uptake and percolation loss of P do change in relation to labile and stable P-pools from acquiring to recycling systems. Moreover the effect of man-made disturbance through acidification or liming is being studied. We perform mesocosm experiments, which represent the relevant aspects of forest ecosystems such as the natural aggregate structure and a natural vertical sequence of organic and mineral soil layers, which allows for adequate quantification of P-storage, sorption kinetics, and mobilization. The exchange rate between the P–stores and roots, mycorrhizal hyphae and soil water is assumed to decrease from the humus layer and / or small biotic aggregates in the Ah to large abiotic aggregates in the Bw horizon. The experiments will be divided in three parts. The first part will be the “control”, where the whole span of the pedogenetic gradient from acquiring to recycling systems will be represented (3 soils that developed from silicate rock with increasing soil acidification, Bad Brückenau, Vessertal, Lüss). The “liming” treatment is expected to result in increased solute concentrations of dissolved organic P (DoP), as well as in increased colloid load. The same is expected to occur as a result of heavy acidification pulses through mineralization of organic macro molecules.

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

To gain further insight into the effects that experimental treatments, tree species, and microflora have on the availability of P and on fluxes between pools of different availability, P in these pools are quantified at the beginning and end of mesocosm experiments and thus facilitate a differentiated assessment of P-uptake and P-mobilization efficiency. Shifts from inorganic to organic P and shifts between the different inorganic P fractions are largely driven by biological processes and also by soil chemical and physical changes. Recent studies have shown, that noticeable changes between these pools can occur over few decades of forest development.

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

This package is worked on by Sonia Meller in frame of the project of Dr. Luster at the Swiss Federal Research Institute WSL.