Phosphorus dynamics during pedogenesis on serpentinite

Many studies on the dynamics of P deal with soils in which calcium and pedogenic iron oxides are the most important factors controlling P fate, whereas serpentinitic soils are characterized by the presence of lithogenic iron oxides and low amounts of calcium in the soil solution. The aim of this work was to investigate the dynamics of P in a soil catena on serpentinite using P fractionation and P sorption data. All soils show P enrichment with respect to the parent material, but the lowest enrichment indices are found in the horizons or profiles with the poorest drainage conditions. The total P content is correlated with both organic matter and pedogenic iron oxides (R = 0.72). The fractionation of P showed that the prevailing forms are those extracted with Na-dithionite-citrate-bicarbonate (DCB), confirming the role of iron oxides in preventing P leaching. However, the nonextractable P forms, determined at the end of the fractionation, account for a large percentage of total P. This is probably caused by the presence of lithogenic magnetite because the organic P content is too small to fully explain the residual P. The parameters of the fitted Freundlich isotherms confirmed the high phosphate sorption capacity of the Fe oxides, both pedogenic and lithogenic, as shown by the multiple regression between the Freundlich K(f) and DCB-extractable iron, magnetite, and serpentine (R2 = 0.99). Serpentine, however, is of only minor importance and shows a lower affinity and a lower capacity for P sorption with respect to iron oxides.