Origin and fate of phosphorus in the Seine watershed (France): Agricultural and hydrographic P budgets

Phosphorus (P) sources (point and diffuse) in the human‐impacted Seine basin (64,840 km 2 ) were evaluated for the year 2000. An agricultural P budget showed that fertilizers represented 59% (20–25 kgP ha −1 ) of P inputs to the soils. A P surplus (2.5 kg P ha −1 y −1 ) contributed to P enrichment of the agricultural soils whose stocks amounted to 1800–5000 kgP ha −1 . A hydrographic P budget showed that runoff (0.39–0.51 kgP ha −1 ) dominated the diffuse sources. These losses represented a very low percentage (0.01%) of the P stocks in soils and contributed to 19–25% of the total P inputs to the drainage network. Point sources remained the main source of P (75–81%), particularly in the downstream urbanized zone. Phosphorus retention in the river drainage network accounted for 9–15% of the total P inputs, indicating that it must not be ignored in large river P budget calculations. The Seine basin exported 8000 tP y −1 (44% as particulate P) to its estuary. The annual mean particulate P in suspended sediment at the outlet (2.9 gP kg −1 ) was fourfold greater than in headwaters and in rural zones. The similar increase of the particulate inorganic P/particulate organic P ratio in suspended sediment along the river continuum clearly indicated the increasing pressure of point sources. The close relation between P content of suspended sediment during the high‐flow period and the P content of agricultural soils resulted in proposing a novel method to calculate the PP losses from runoff.