Distinct export dynamics for dissolved and particulate phosphorus reveal independent transport mechanisms in an arable headwater catchment
Abstract
This paper investigates particulate phosphorus (PP) and soluble reactive phosphorus (SRP) concentrations at the outlet of a small
(5 km²) intensively farmed catchment to identify seasonal variability of sources and transport pathways for these two phosphorus
forms. The shape and direction of discharge-concentration hystereses during floods were related to the hydrological conditions in
the catchment during four hydrological periods. Both during flood events and on an annual basis, contrasting export dynamics
highlighted a strong decoupling between SRP and PP export. During most flood events, discharge-concentration hystereses for
PP were clockwise, indicating mobilization of a source located within or near the stream channel. Seasonal variability of PP
export was linked to the availability of stream sediments and the export capacity of the stream. In contrast, hysteresis shapes for
SRP were anticlockwise, which suggests that SRP was transferred to the stream via subsurface flow. Groundwater rise in wetland
soils was likely the cause of this transfer, through the hydrological connectivity it created between the stream and P-rich soil
horizons. SRP concentrations were the highest when the relative contribution of deep groundwater from the upland domain was
low compared with wetland groundwater. Hence, soils from non-fertilized riparian wetlands seemed to be the main source of
SRP in the catchment. This conceptual model of P transfer with distinct hydrological controls for PP and SRP was valid
throughout the year, except during spring storm events, during which PP and SRP exports were synchronized as a consequence
of overland flow and erosion on hillslopes.