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

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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.