Patterns and processes of phosphorus transfer from Irish grassland soils to rivers—integration of laboratory and catchment studies

The objectives of this research were to investigate the patterns of phosphorus (P) transfer from fertilised soils to streams and to investigate the processes responsible for such losses. The three lowland grassland catchments involved in the study were: the Oona Water, Co. Tyrone, the Dripsey, Co. Cork and the Clarianna, Co. Tipperary. In each catchment, three or four nested sub-catchments were monitored for flow and P fraction concentrations at scales from 0.15 to 88.50 km2. The P sorption and desorption characteristics of soil samples from each catchment were also determined. Results for 2002 indicated that the reactive P transfer, especially at the small catchment scale, could be predicted by Langmuir soil P properties, especially in the non-calcareous soils. In these soils, although iron (Fe) was an important determinant of P sorption, the sorption sites were weakly bound and binding energy only increased with increasing aluminium (Al) concentration. In calcareous soils, the Langmuir model was not applicable and P retention in these soils, which resulted in low reactive P transfer to streams, was most likely linked to P and calcium (Ca) precipitation and dissolution reactions. Particulate P transfer from all soils reflected differences in soil hydrological properties when flow was summarised as the Q5:Q95 ratio, which is a metric of runoff flashiness. Reactive and particulate P patterns changed with increasing catchment area due to changing hydrology and the cumulative influence of point source inputs. The findings of this study, therefore, implicated soil type, flow regime and scale as the important factors when linking P transfer process to catchment patterns and will be central to developing monitoring and mitigation strategies for managing P transfers to freshwater.