Phosphate and Sulfate Sorption in Spodosols with Albic Horizon from Northern Spain

Abstract

The affinity of PO4 and SO4 for reactive Al and Fe compounds may differ between the Bh and Bs horizons of Spodosols. Sorption of SO4 and PO4 in soils was investigated in four Spodosols from Northern Spain. Sorption experiments were carried out by adding 1.6 and 16.1 mM KH2PO4 or 0.4 and 1.6 mM K2SO4 to soils (soil/solution, 1:10). Most (98.5–99.8%) of the PO4 added at 1.6 mM was retained by the illuvial Bh, Bhs, Bsh, and Bs horizons. The SO4 retained by the same horizons at the same initial concentration was below 28%. Phosphate sorption at 16.1 mM in the Bh and Bhs horizons was significantly (P < 0.05) greater than in the Bsh and Bs, corresponding to their greater oxalate extractable‐Al and –Fe contents, whereas the opposite was observed for SO4 absorption. Sorption of PO4 primarily displaced mineral‐bound organic C and, to a lesser extent, Si into solution, whereas sorption of SO4 increased solution pH. The molar ratios of organic C released into solution and PO4 sorbed were greater in the Bsh and Bs horizons than in the Bh and Bhs horizons. The differences observed in PO4 and SO4 sorption between Bh and Bs horizons were mainly attributed to the different organic C contents of the horizons and to the dissimilar affinities of the inorganic anions studied for reactive sites.