Aluminum Forms in Solid Phase of Soils Developed over Schists as a Function of Land Use

Abstract

ABSTRACTLand use may modify certain soil properties while soil physicochemical characteristics can influence metal partitioning in soils. Therefore, the total content and various forms of aluminum (Al) in solid phase of schist-developed topsoils (0–20 cm) in NW Spain under different land uses (i.e., forest, pasture, and cultivation) were evaluated to identify the Al-bearing phases. Aluminum fractionation was performed, using a six-step sequential extraction procedure with ammonium acetate, hydroxylamine hydrochloride, ammonium oxalate in darkness, hydrogen peroxide, ammonium oxalate under ultraviolet radiation, and acid digestion. Mean concentrations of total Al were similar in the soils under three land uses. Mean percentage of the various Al forms in all soils were in the following order: residual fraction > amorphous compounds > crystalline compounds > water-soluble/exchangeable/specifically adsorbed > bound to oxidizable organic matter > manganese oxides. The forest soils contained considerably higher contribution of amorphous compounds (16.3%) to total Al concentration compared with the soils under other two uses (mean about 9%). Maximum mean concentration of exchangeable Al was also observed in forest soils (mean 8.8% of total Al vs. about 4% in pasture soils and cultivated soils); this is attributed to lower pH and higher organic matter content of the forest soils. Thus, this study revealed the impact of land use on the Al-bearing phases and, hence, in its bioavailability to plants.