Influence of higher rain intensities on phosphorus movements in the upper half meter of macroporous clay soil

In climate change scenarios, the frequency of high-intensity rain events in Sweden is assumed to increase. In a plot experiment at Ultuna, Uppsala, the influence of rain intensities on phosphorus (P) transport in the uppermost 0.5 m of a clay soil was studied at 16 locations. A rain simulator, 0.5 × 0.5 m and mounted 1 m above the soil surface, was used to simulate 85–500 min rain sequences causing small (4–9 mm h⁻¹) and large (22–28 mm h⁻¹ and one extreme at 37 mm h⁻¹) steady water fluxes (intensity) in the underlying soil profile. Water percolated to a zero-tension collector tray at 0.5 m depth where drain water and its sediment load was sampled at discrete time intervals. The total P (TP) mass flux ranged, at low intensity, between 12–92 μg m⁻² min⁻¹ (average 28.1 μg m⁻² min⁻¹) and, at high intensity, between 83–375 μg m⁻² min⁻¹ (average 168.5 μg m⁻² min⁻¹) and 648 μg m⁻² min⁻¹ at the extreme intensity. The soluble reactive (inorganic) P (SRP) mass flux ranged, at low intensity, between 1–65 μg m⁻² min⁻¹ (average 10.0 μg m⁻² min⁻¹) and, at high intensity, between 6–205 μg m⁻² min⁻¹ (average 47.9 μg m⁻² min⁻¹) and 495 μg m⁻² min⁻¹ at the extreme intensity. Thus, in the intensity range 4–28 mm h⁻¹, TP and SRP increased, on average, by approximately 12% (μg m⁻² min⁻¹) per unit increase in intensity (mm h⁻¹). The results of this study demonstrate increased sediment and P loss/mobility for clay soil under increased precipitation intensity predicted under climate change.