Variability of phosphorus sorption properties in hydromorphic soils: consequences on P losses in agricultural landscapes

International audiencencreasing concerns over water eutrophication due to agricultural phosphorus (P) loss have led to the development of indicators to assess the risk of P release from agricultural soils. Recently, a logarithmic equation linking the degree of phosphorus saturation (DPS) to the simple water-soluble P (WSP) content of soils has been proposed as a universal method to assess this risk based, however, mainly on the analysis of well-drained soils. Here, we studied the P sorption properties and DPS values of 69 hydromorphic soils from cultivated and uncultivated wetland zones located in Brittany, Western France, to test whether the method could also apply to poorly-drained soils. The bulk soil analysis showed that P contents of the studied hydromorphic soils were 30 to 80% higher than P contents normally found in Brittany soils, evidencing a possible P enrichment process. Adsorption isotherms revealed a surprisingly high variability in the P sorption properties as a function of the location of the soil (maximum P adsorption capacity ranging from 500 to 1850 mg kg-1), which is caused by variations in the phases controlling P sorption in soil (from clay to organic matter and/or iron and aluminum oxides, depending on the soil location). Distinct relationships between DPS and WSP values were also obtained depending on the location of the soils. The obtained DPS vs. WSP relationships showed that the P saturation threshold above which the risk of dissolved P release increases markedly is 30% lower on average for hydromorphic soils than for well-drained soils. Hydromorphic soils appear to be more at risk of releasing dissolved P at the same DPS values than well-drained soils. The present study indicates an underestimation of the P release risk from hydromorphic soils by the existing method developed for well-drained soils and calls for the development of specific risk assessment tools for hydromorphic soils, especially given on the strong spatial heterogeneity of their P sorption propertie