Isotope geochemical determination of phosphorus weathering sources and fluxes in forest ecosystems

磷是所有陆地生态系统必需的营养物质，其主要来源是岩石风化释放的磷。由于岩石风化本身是一个缓慢的过程，向生态系统提供磷的速度最终受到岩石风化速度和土壤保留释放磷的能力的限制。森林生态系统通过提高磷的循环速率来应对供应缓慢。因此，要对生态系统中的磷循环进行全面量化，就需要知道这些初始释放速率。然而，这样的费率仍未普遍实现。该项目将利用若干创新的同位素地球化学工具，在DFG优先方案“生态系统营养”的四个高地“ 2级”地点确定这些通量。1)在两个花岗岩/片麻岩基岩点，将结合元素质量平衡，利用稀有的原位宇宙成因同位素10Be测定土壤中磷通量，定量测定土壤中磷的枯竭。2)在火山基底下的两个地点，不包含测量现场生产的10Be所需的石英，我们将使用一个新的系统来获得相同的参数：10Be（陨星）/9Be同位素比值结合风化质量平衡。3)测定磷灰石等原生含磷矿物、土壤水体、土壤交换相、树木茎、木、叶中87Sr/86Sr同位素比值，确定磷源。为了评估磷在土壤中的滞留行为，我们将把这些主要通量与河流和地下水的磷排放以及在其他SPP项目中确定的这种排放的优先流动路径进行比较。最终，我们将关闭磷通量循环，通过比较凋落物通量和磷风化通量来确定生态系统的磷循环效率，并通过比较溶解磷排放量和风化主要输入通量来确定通量和循环对变化的敏感性。

The primary source of phosphorus, an essential nutrient to all terrestrial ecosystems, is its release by weathering of rock. As rock weathering is an inherently slow process, the rate at which phosphorus is supplied to ecosystems is ultimately limited by the rock weathering rate, and by the ability of soils to retain the released phosphorus. Forest ecosystems respond to slow supply by increasing their phosphorus recycling rate. Hence a full quantification of the phosphorus cycle through ecosystems requires knowing these initial release rates. Yet such rates are still not generally available. This project will make use of several innovative isotope geochemical tools to determine these fluxes at four upland “Level 2” sites of the DFG Priority Programme “Ecosystem Nutrition”. 1) In two sites of granitoid/gneissic bedrock, the rare in situ-produced cosmogenic isotope 10Be will be used in combination with element mass balances to determine phosphorus fluxes and quantify phosphorus depletion in soil. 2) In two sites underlain by volcanic substrates that do not contain the quartz required for measuring in situ-produced 10Be we will use the a new system to obtain the same parameters: the 10Be(meteoric)/9Be isotope ratio in combination with weathering mass balances. 3) We will measure the radiogenic 87Sr/86Sr isotope ratio in primary phosphorus-containing minerals such as apatite, in soil water, in the exchangeable soil phase, and in tree stem wood and leaves to determine phosphorus sources. To evaluate the retention behavior of phosphorus in soils we will compare these primary fluxes with the phosphorus discharge by streams and groundwater and the preferential flow paths this discharge takes as determined in other SPP projects. Ultimately, we will close the phosphorus flux loop determine the ecosystems‘ phosphorous recycling efficiency by comparing litterfall fluxes with phosphorus weathering fluxes, and the sensitivity of fluxes ad recycling to change by comparing the dissolved phosphorus discharge with the primary input flux through weathering.