The impacts of exceptional rainfall on phosphorus mobilisation in a mountain agroforestry catchment (NE, Spain)

Abstract

Erosion causes significant soil and nutrient losses that can reach streams and degrade habitats. Phosphorus (P) is among the nutrients of greatest concern for water pollution. Due to the increase in the number of storm events over the last decade, which could rise further under climate change scenarios, a more in-depth analysis of the effect of rainfall on the behaviour of P in fragile environments is needed. Little is known about the mobilisation and export of P in mountainous Mediterranean agroecosystems. To contribute to this knowledge, this research analysed the variability of P in the sediments of streambeds of different orders in an agroforestry area of the Northern Ebro Basin (Aragón, Spain) following an exceptional rainfall event; the implications of different land uses were also explored. Sediment composition was assessed before and after the rainfall event in three nested subcatchments and then related to soil properties. Phosphorus was mostly linked to the mineral fraction (mainly to silicates), while the links between P with clay and organic matter (P-clay, P-OM) were very weak. The P-OM links occurred only in the soils of forested areas. Agricultural lands, which are prone to erosion and had the highest P concentrations, contribute to P release. However, the streambeds and the lateral erosion of channel banks by floods triggered by the rainfall event should be considered as the main contributors to the export of P. The high intensity rainfall event led to 35% and 60% reductions in clay and OM, respectively, and to an enrichment of P in the sediments, the concentrations of which were lower in the headwaters than downstream. This means that the P in streambeds remains exposed in relatively high concentrations following extreme rainfall events with implications for the P cycle and water pollution.