Phosphorus budget in the Marne Watershed (France): urban vs. diffuse sources, dissolved vs. particulate forms
Résumé
We evaluated the P sources (point, diffuse), through a nested watershed approach investigating the Blaise (607 km2), dominated by livestock farming, the Grand Morin (1202 km2), dominated by crop farming, and the Marne (12,762 km2), influenced by both agriculture and urbanization. Fertilizers account for the main P inputs (>60%) to the soils. An agricultural P surplus (0.5–8 kg P ha–1 year–1) contributes to P enrichment of the soil. The downstream urbanized zone is dominated by point sources (60%, mainly in dissolved forms), whereas in the upstream basin diffuse sources dominate (60%, mostly particulate). Among the diffuse sources (losses by forests, drainage and runoff), losses by runoff clearly dominate (>90%). P retention in the alluvial plain and the reservoir represents 15–30% of the total P inputs. Dissolved and particulate P fluxes at the outlet of the Marne are similar (340 and 319 tons of P year–1, respectively). The Blaise sub-basin receives P from point and diffuse sources in equal proportions, and retention is negligible. The Grand-Morin sub-basin, influenced by the urbanized zone receives, as does to the whole Marne basin, 60% of P inputs as point sources. The total particulate phosphorus in suspended sediments averaged 1.28 g P kg–1, of which about 60% are inorganic and 40% organic P. Particulate phosphorus exchangeable in 1 week and 1 year (32P isotopic method) accounts for between almost 26% and 54% of the particulate inorganic phosphorus in the suspended sediment and might represent an important source of dissolved P, possibly directly assimilated by the vegetation.