Phosphorus and water flux dynamics in runoff and plant uptake in forested headwaters

水文通道构成了磷动员源与磷向河流输出之间的关键环节。在样地尺度上，磷的运移过程相对较好，然而，由于运输尺度和过程的复杂性以及不同的磷分离机制，对坡面和水源磷运移的认识受到限制。土壤中的横向地下流可以为磷的输出贡献大量的磷通量，因为磷的运输通常与快速流动过程有关，特别是森林山坡是快速流动通常发生的景观单元。因此，在评估磷运输动力学时，对山坡水文动力学（包括深层渗流和地下水流动以及植物对磷的吸收）的良好过程知识非常重要。在这项以山坡和水源磷动力学为重点的实验和模型研究中，我们将根据SPP的一般思想，即土壤和地质中的磷耗竭驱动森林生态系统从磷获取系统向磷回收系统的演变，研究水文径流过程对森林流域磷运输的影响。在第一阶段的三个站点建立一个广泛的山坡观测平台，我们将使用新颖的方法和高频采样来捕捉水和磷通量的高时空动态。我们假设在极端降雨事件或潮湿条件下，磷耗竭的恢复在获取系统中比在回收系统中更快。从不同土壤深度向树木吸收磷和水在磷循环系统中比在获取系统中具有更强的解耦性。最后，生物活性土壤带微生物P矿化的P淋溶仅限于土壤内部水流快速的条件下，而矿物风化的P则根据下伏基岩地质情况，通过深层渗流和地下水更连续地流失。这些假设将在SPP的三个核心站点进行测试，使用一个复杂的、连续的监测系统，以高时间分辨率监测植物、土壤和地下水中的径流和磷运输。基于事件和连续采样的P物种，稳定的水同位素和其他地质示踪剂将使我们能够得出与P通量和P输运过程相关的水年龄和传递时间分布。最后，我们将进一步开发一个基于过程的山坡模型，模拟不同的流动和运输路径，将径流和磷的内部结构和动态与山坡和集水区的特性联系起来。

Hydrological pathways form the critical link between the source of P mobilization and the P export to streams. The P mobilization processes at the plot scale are comparatively well understood, however, the knowledge of P delivery through hillslopes and headwaters is limited by the complexities of the transport scales and processes involved and the different P detachment mechanisms. Lateral subsurface flow in the soil can contribute large P fluxes to the P export, because P transport is often connected with fast flow processes, and in particular forested hillslopes are landscape units where fast flow typically occurs. Sound process knowledge of hillslope hydrological dynamics including deep seepage and groundwater flow and P uptake to plants is thus highly important when assessing P transport dynamics. In this experimental and modeling study focusing on hillslope and headwater P dynamics, we will study the effects of hydrological runoff processes for the P transport in forested catchments following the general idea of the SPP that the P depletion in soils and geology drives the evolution of forest ecosystems from P-acquiring systems to P-recycling systems. Building on a extensive hillslope observation platform at three sites from the first phase, we will use novel methods and high frequency sampling to capture the high temporal and spatial dynamics of water and P fluxes. We hypothesis that the recovery of P depletion during extreme rainfall events or wet conditions is faster in acquiring systems than in recycling. The uptake of P and water into trees from different soil depths is stronger decoupled in P recycling systems than it is in acquiring systems. And finally, P leaching from the microbial P mineralization in the biologically active soil zone is restricted to conditions with fast soil-internal water flow whilst P from mineral weathering is lost more continuously through deep seepage and groundwater depending on the underlying bedrock geology. These hypotheses will be tested at three core sites of the SPP with a sophisticated, continuous monitoring system for runoff and P transport in the plants, soil and groundwater at high temporal resolution. Event-based and continuous sampling for P species, stable water isotopes and other geogenic tracers will allow us to derive water ages and transit time distributions to be linked with P fluxes and P transport processes. Finally, we will further develop a process-based hillslope model simulating the different flow and transport pathways to link the internal structure and dynamics of runoff and P to the hillslope and catchment properties.