Lateral Transport of phosphorous along hillslopes and its relation with water age

Phosphorous (P) and nitrogen (N) are considered the nutrients to most frequently limit the productivity of both freshwater (Hecky & Kilham (1988), Elser et al. (1990)) and terrestrial ecosystems (Walker & Syers (1976), Vitousek & Howard (1991)). Both nutrients play a crucial part in human agriculture accordingly. While the N-cycle is dominated by atmospheric deposition which is mainly due to human activity (Vitousec (1997)), P input into ecosystems is mainly cause by weathering of bedrock material and atmospheric dust with far less anthropogenic impact (Buendia et al. (2010)). This difference in the replenishment process of both nutrients leads to a different pattern of availability in both spatial and temporal terms. While N is continuously supplied from the atmosphere, P needs to be transported from the zone of mineral weathering to the biosphere via roots or lateral water flow. Therefore, P is supposed to be found on the weathering interface of the bedrock as well as in the in the living and decaying biomass. The further soil development has progressed, the more troublesome it gets for the biosphere to transport new P via roots to the zone of biological activity. Therefore, recycling of P that already is in the biomass becomes increasingly important with progressing soil development. After the weathering progresses so far belowground that no P can be uplifted to the zone of biological activity, a “terminal steady-state” (Walker & Syers, 1976) or “recycling system” is pronounced, where growing biomass can supply itself only with P originating from decaying biomass. It is suspected that P losses from such a system occur mainly via biomat- and fast subsurface flow in contrast to (agricultural) soil with a rather high P content, where P losses are more present with increasingly pronounced matrix flow (Haygarth et al. (1998)).

Since the main areas of P turnover in the soil – root channels and the soil-bedrock-interface – are hydrological pathways as well, it is reasoned that hydrological pathways form the critical link between the source of P mobilization and the P export to streams (Heathwait & Dills, 2000). The export of P via hydrological systems depends on the present type of land-cover, the individual local characteristics regarding spatiotemporal rainfall patterns and preexisting hydrological networks as well as antecedent soil moisture and mean travel time of incoming water through the catchment (Heathwait & Dills, 2000). Furthermore, the bonding form of P has a large influence on the effectiveness of certain hydrological flowpaths regarding P export, as for example colloidal P can only be transported in preferential flowpaths whereas solute P can be transported in diffuse matrix flow.

It is the overall goal of this research project to identify controls for lateral transport of phosphorous along hillslopes and its relation to water age. For this purpose, discharge in the headwater, slope groundwater and subsurface flow will be assessed regarding their magnitude, physical and chemical characteristics including the P content. This will serve foremost to establish basic knowledge about the magnitude and temporal dynamics of P-transport in forested headwaters via these hydrological subsystems. In combination with water age data derived from stable isotope measurements, a process based model of the hillslope hydrology will be created based on the Hill-Vi modelling software (Weiler and McDonnell (2004, 2006)). This will allow for the prediction of P export as a function of catchment characteristics and –state which allows for spatial and temporal upscaling and therefore more general conclusions about the extent of P export from forested headwater catchments.