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

水文通道是磷迁移来源和磷向河流输出之间的关键环节。小区尺度上的磷迁移过程已被较好地理解，然而，由于涉及的迁移尺度和过程的复杂性以及不同的磷分离机制，对磷通过山坡和水源的输送的了解是有限的。土壤中的横向地下径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，特别是森林覆盖的山坡是通常发生快速流动的景观单元。因此，对包括深层渗漏、地下水流动和植物对磷的吸收在内的坡面水文动力学的可靠过程知识在评估磷的运输动态时是非常重要的。在这项以山坡和水源磷动态为重点的实验和模拟研究中，我们将遵循SPP的基本思想，即土壤和地质中的磷耗竭推动森林生态系统从磷获取系统向磷循环系统的演变，研究水文径流过程对森林流域磷迁移的影响。在第一阶段的三个地点建立一个广泛的山坡观测平台，我们将使用新的方法和高频采样来捕捉高时空动态的水和磷的通量。我们假设，在极端降雨事件或潮湿条件下，收集系统中磷耗竭的恢复速度快于循环。在磷循环系统中，树木对不同土壤深度的磷和水分的吸收比在获取系统中更具解偶性。最后，生物活性土壤中微生物磷矿化的磷淋失仅限于土壤内部水流较快的条件，而矿物风化的磷则更多地通过深层渗漏和地下水流失，具体取决于下伏的基岩地质条件。这些假设将在SPP的三个核心地点进行验证，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磷物种、稳定水同位素和其他地质示踪剂将使我们能够得出与磷通量和磷迁移过程相联系的水龄和渡越时间分布。最后，我们将进一步开发一个基于过程的山坡模型，模拟不同的径流和输送路径，将径流和磷的内部结构和动态与山坡和集水特性联系起来。

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.