Source areas of phosphorus transfer in an agricultural catchment, south-eastern Norway

The strategy to mitigate phosphorus (P) losses in areas of arable cropping in Norway has focused on measures to reduce erosion. Risk assessment of erosion has formed the basis for implementation of the measures. The soil P content has increased during recent decades, motivating an evaluation of its effect on P transfer in the landscape. The present study describes the spatial variability of runoff P concentrations from an agricultural dominated catchment (4.5 km²), representative for agriculture in south-eastern Norway. The concentrations of suspended sediments (SS), total P (TP) and dissolved reactive P (DRP) in runoff from 22 subcatchments (0.3–263 ha) during one year (monthly and during runoff-events) were evaluated. Contributions from point sources were 38 kg TP yr^–1 compared to a total P loss of 685 kg yr^–1 from the whole catchment. During low flow, mean diffuse TP concentration in runoff from subcatchments varied from 28 to 382 µg l^–1. The mean low flow TP concentration was 39 µg l^–1 from the housing area (only diffuse runoff) and 33 µg l^–1 from the forested area. During high flow the highest diffuse TP concentration was measured in an area with high erosion risk and high soil P status. At the subcatchment level the transfer of SS varied from 25 to 175% of the whole catchment SS transfer. Correspondingly for TP, the transfer varied from 50 to 260% of the whole catchment TP transfer. For each of five agricultural subcatchments the slope of the relationship between TP and SS concentrations reflected the mean soil P status of the subcatchment. Erosion risk estimates were closely related to the SS concentration (R²=0.83). The study illustrates that soil P status in addition to soil erosion is an important factor for P transfer.