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 in vestigate the relationships between SO42- sorption and contents of Al and r e compounds in soils from Galicia, NW Spain, with andic soil properties. Ni neteen soils derived from gabbros, amphibolites, basic granulites, biotitic schists, and amphibolitic phyllites were sampled. Sulfate sorption was stu died by adding a 0.4 mM SO42- solution (initially acidified to pH 3.0) to s oils. Aluminum-humus complexes were abundant in surface horizons. Oxalate- and pyrophosphate-extractable Al (Al-o and Al-p, respectively) decreased wi th depth, whereas there was an increase in the NaOH-extractable Al (Al-n) m inus Al-o fraction, associated with an increase in poorly crystalline forms of both gibbsite and 1:1 phyllosilicates. Iron was mostly present as cryst alline oxyhydroxides. The abundance of variable charge materials and the ac idity of these soils (pH H2O < 5.7) explain the high SO42- sorption detecte d. Sulfate sorption was positively correlated (at P < 0.01) with Al-n, Al-o , and Al-p in surface horizons, and with Al-n, Al-o, Fe extractable with di thionite-citrate (Fe-d) and oxalate (Fe-o) in subsurface horizons. The corr elation between SO42- sorption and Al-o decreased sharply when data from al l 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 bet ween the Al/Fe fractions and SO42- sorption. The results emphasize the high SO42- retention capacity of both surface and subsurface horizons of nonvol canic Andisols and Andic soils front Galicia. (C) 2001 Elsevier Science B.V . All rights reserved.