Phosphorus losses from two representative small catchments in the Mediterranean part of Spain

Information about phosphorus (P) losses from agricultural catchments in Mediterranean environments is scarce. In this work, P losses in overland flow from two representative small Mediterranean catchments, one dominated by Alfisols and the other by Vertisols, were studied. At the lowest level of each catchment, overland flow was measured and several runoff samples taken in each runoff event during two growing seasons (2001-2002 and 2002-2003). After centrifugation, total P in sediments and total and molybdate reactive P in supernatant were determined. Different chemical extraction methods were used to quantify the forms of P in soils and sediments. Total P losses in the studied catchments ranged between 0.5 and 3.2 kg ha(-1) year(-1), losses higher than 2 kg P ha(-1) being observed in one event. Phosphorus was mainly lost in the sediments, the ratio of total dissolved P to particulate P being higher in the Alfisol than in the Vertisol catchment. Phosphorus concentration in sediments from the Vertisol catchment was similar to that in the source soils, whereas sediments in the Alfisol catchment had 2.1 times more inorganic P and 9 times more organic P (OP) than the source soils. In the latter catchment, there was an enrichment in the more labile inorganic P forms in the sediments relative to the source soils, which corresponds to a relative enrichment in iron (Fe) oxides by a factor of 2.4. Alfisols had lower TP contents and exhibited lower erosion rates than Vertisols in the studied period but they posed a greater environmental risk than the latter soils because their sediments were richer in P and had a higher proportion of P in forms bound to the Fe oxides compared to the source soils-these P forms can be easily released with the onset of reducing conditions at the bottom of waterbodies. A study of the P enrichment ratios and the dominant P forms in eroded sediments is therefore necessary to predict the impact of P losses from soils on the ecological quality of waterbodies.