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

1. 在森林土壤的发育过程中，磷的丰度和有效性普遍下降，从而限制了生物活动。森林生态系统通过发展有效的磷循环机制来适应这一过程，包括形成稳定的磷库和优化的树木对磷的吸收。在中欧，这种敏感的演变受到人为沉积导致的加速土壤酸化的干扰。在不同森林中观察到的磷营养的快速下降支持了这一假设。该项目考察了挪威云杉和欧洲山毛榉的磷吸收和动员效率如何沿着土壤生态梯度增加，范围从具有高初级矿物磷植物有效性的“年轻”土壤到具有低矿物磷有效性的“年老土壤”。我们进一步感兴趣的是i)物理和化学边界条件对磷循环效率的影响，即保持树木的磷有效性，同时尽量减少磷淋溶；ii)通过氮肥人工加速酸化或通过实验石灰强化酸缓冲将如何变化，或可能干扰P的吸收和动员效率。我们想评估以下竞争过程：i)土壤中磷的储存/固定，ii)随着渗水输出磷，以及iii)树木通过根和菌根菌丝与菌根圈土壤之间的相互作用开采土壤磷资源。这三个过程在天然林土壤中都表现出明显的空间异质性，我们的目标是在实验中保持这种异质性，以便能够评估它们在生态系统水平上的相关性。材料和方法在温室中对SPP核心地点和1 ~ 2年生云杉和山毛榉幼苗的土壤进行土壤同化试验：i)通过测量土壤中磷的组成和分布、土壤水分的通量和植物磷的吸收，在自然结构的土壤柱中评估与总磷和稳定磷库相关的植物吸收和渗透损失沿磷枯竭梯度的变化。有和没有覆盖有机表层的不同处理旨在评估有机土壤和矿质土壤对磷营养的相对作用。研究了石灰和氮肥对酸性磷耗竭梯度两个极端点的扰动。ii)通过比较一个酸性场地B层的自然结构土壤和不同基质电位下的筛分土壤，研究了矿质土壤团聚体结构和通气性对磷吸收和动员效率的影响。iii)在根箱中对主生根区筛选过的土壤进行试验，研究植物诱导菌根际在酸化/碱化、潜在磷动员物质的发生、磷动员和微生物群落组成等方面的异质性。

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.