Extractability of aluminium and forms in solution of soils derived from granite in Portugal

Extractable aluminium (Al) was determined in surface and subsurface horizons of eleven pedons derived from granite occurring in Portugal, under a wide range of precipitation. Five successive extractions with 1 M KCl, 0.5 M CuCl2 and 0.33 M LaCl3 were used. Monomeric (Al m) and organic (Al org) forms of Al in the soil solution, extracted at field capacity, were also determined. Amounts of extracted Al were greatest with the 0.5 M CuCl2 and lowest with the 1 M KCl. These amounts showed strong positive correlation with soil organic C contents. Soil horizons having high contents (56-80 g kg-1) of organic C showed the highest amount of extractable Al by the 0.5 M CuCl2 (12.2-30.9 cmol c kg-1). Organically bound Al extracted by the 0.5 M CuCl2 and 0.33 M LaCl3 was also highest in these soils (7.6-10.1 and 4.7-8.3 cmol c kg-1, respectively). In contrast, lowest amounts of Al extracted by the 1 M KCl, 0.5 M CuCl2 and 0.33 M LaCl3 (0.9-4.3, 4.1-17.1 and 2.2-7.2 cmol c kg-1, respectively) were observed in horizons containing low organic C content (3-22 g kg-1). Concentrations of total (AlT), monomeric (Al m) and organic (Al org) Al in the soil solution of studied soils also increased with increasing soil organic C content, but only Al m was significantly correlated with soil organic Ccontent. Solution of soils having low organic C contents (3-6 g kg-1) showed the lowest concentrations of AlT, Al m and Al org (0.09-1.60, 0.06-0.38 and 0.03-1.29 µg mL-1, respectively). In contrast, the concentrations of AlT (7.1-17.2 µg mL-1),Al m (0.8-0.9 µg mL-1),and Al org (7.6-16.9 µg mL-1)were much higher in soils containing high amounts of organic C (37-73 g kg-1). In order to monitor the effects of Al in the soil and quality of sub-superficial water, changes in the concentration of monomeric forms of Al and their activities in the soil solution need further study.