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

The affinity of PO 4 and SO 4 for reactive Al and Fe compounds may differ between the Bh and Bs horizons of Spodosols. Sorption of SO 4 and PO 4 in soils was investigated in four Spodosols from Northern Spain. Sorption experiments were carried out by adding 1.6 and 16.1 m M KH 2 PO 4 or 0.4 and 1.6 m M K 2 SO 4 to soils (soil/solution, 1:10). Most (98.5–99.8%) of the PO 4 added at 1.6 m M was retained by the illuvial Bh, Bhs, Bsh, and Bs horizons. The SO 4 retained by the same horizons at the same initial concentration was below 28%. Phosphate sorption at 16.1 m M 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 SO 4 absorption. Sorption of PO 4 primarily displaced mineral‐bound organic C and, to a lesser extent, Si into solution, whereas sorption of SO 4 increased solution pH. The molar ratios of organic C released into solution and PO 4 sorbed were greater in the Bsh and Bs horizons than in the Bh and Bhs horizons. The differences observed in PO 4 and SO 4 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.