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

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1. RationaleDuring 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 sensitive evolution is disturbed by an accelerated soil acidification driven by anthropogenic deposition. The fast decrease in P nutrition observed in various forests supports this hypothesis.2. ObjectivesThe project examines for Norway spruce and European Beech, how P uptake and mobilization efficiency increase along a soil ecological gradient ranging from “young” soils with high plant availability of P in primary minerals to “aged soils” with reduced availability of mineral P. We are further interested in i) the influence of physical and chemical boundary conditions on the efficiency of P-recycling, i.e. maintaining P availability for trees while minimizing P-leaching, and ii) how artificially accelerated acidification through nitrogen (N) fertilization or enhanced acid buffering through experimental liming will vary, or possibly disturb P uptake and mobilization efficiency. We want to assess the following competing processes i) storage / fixation of P in the soil, ii) P export with the seepage water and iii) mining of the soil P resources by the tree through interactions between roots and mycorrhizal hyphae with the mycorrhizosphere soil. All three processes display pronounced spatial heterogeneity in natural forest soils, which we aim to maintain in our experiments to such an extent that their relevance at ecosystem level can be assessed.3. Material and MethodsMesocosm experiments with soil from SPP core sites and 1 to 2 year old spruce and beech seedlings will be carried out in the greenhouse: i) changes in plant uptake and percolation loss in relation to total and labile P pools along a P depletion gradient will be assessed in naturally structured soil columns, by measuring composition and distribution of P-species in the soil, Pfluxes in the soil water, and plant P uptake. Separate treatments with and without inclusion of an organic surface layer aim at assessing the relative role of organic and mineral soil for P nutrition. The disturbance by liming and N fertilization is studied for the two extreme sites of the acid P depletion gradient. ii) The impact of aggregate structure and aeration in mineral soils on P uptake and mobilization efficiency is studied by comparing naturally structured soil from the B horizon of one acidic site with sieved soil at different matric potentials. iii) Experiments with sieved soil from the main rooting zone are carried out in rhizoboxes in order to study plant induced heterogeneity in the mycorrhizosphere in terms of acidification/alkalinisation, the occurrence of potentially P mobilizing substances, P mobilization and microbial community composition.