Sulfate sorption in nonvolcanic Andisols and Andic soils from Galicia, NW Spain

Soils with andic soil properties generally retain large amounts of SO42−, a feature related to their elevated contents of short-range ordered minerals and/or insoluble Al–humus compounds. The objective of this study was to investigate the relationships between SO42− sorption and contents of Al and Fe compounds in soils from Galicia, NW Spain, with andic soil properties. Nineteen soils derived from gabbros, amphibolites, basic granulites, biotitic schists, and amphibolitic phyllites were sampled. Sulfate sorption was studied by adding a 0.4 mM SO42− solution (initially acidified to pH 3.0) to soils. Aluminum–humus complexes were abundant in surface horizons. Oxalate- and pyrophosphate-extractable Al (Alo and Alp, respectively) decreased with depth, whereas there was an increase in the NaOH-extractable Al (Aln) minus Alo fraction, associated with an increase in poorly crystalline forms of both gibbsite and 1:1 phyllosilicates. Iron was mostly present as crystalline oxyhydroxides. The abundance of variable charge materials and the acidity of these soils (pH H2O<5.7) explain the high SO42− sorption detected. Sulfate sorption was positively correlated (at P<0.01) with Aln, Alo, and Alp in surface horizons, and with Aln, Alo, Fe extractable with dithionite–citrate (Fed) and oxalate (Feo) in subsurface horizons. The correlation between SO42− sorption and Alo decreased sharply when data from all horizons were pooled. Blockage of reactive surfaces by organic compounds, in surface horizons and, the different Al compounds found in both types of horizons may explain the differences between them in the relationships between the Al/Fe fractions and SO42− sorption. The results emphasize the high SO42− retention capacity of both surface and subsurface horizons of nonvolcanic Andisols and Andic soils from Galicia.