Phosphorus dynamics during pedogenesis on serpentinite

Many studies on the dynamics of P deal with soils in which calcium and pedo genic iron oxides are the most important factors controlling P fate, wherea s serpentinitic soils are characterized by the presence of lithogenic iron oxides and low amounts of calcium in the soil solution. The aim of this wor k was to investigate the dynamics of P in a soil catena on serpentinite usi ng P fractionation and P sorption data. All soils show P enrichment with re spect to the parent material, but the lowest enrichment indices are found i n 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 tho se extracted with Na-dithionitecitrate-bicarbonate (DCB), confirming the ro le of iron oxides in preventing P leaching. However, the nonextractable P f orms, determined at the end of the fractionation, account for a large perce ntage of total P. This is probably caused by the presence of lithogenic mag netite because the organic P content is too small to fully explain the resi dual P. The parameters of the fitted Freundlich isotherms confirmed the hig h phosphate sorption capacity of the Fe oxides, both pedogenic and lithogen ic, as shown by the multiple regression between the Freundlich K-f and DCB- extractable iron, magnetite, and serpentine (R-2 = 0.99). Serpentine, howev er, is of only minor importance and shows a lower affinity and a lower capa city for P sorption with respect to iron oxides.