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The objective of this research was to study the rhizosphere of tree species of acid forest soils derived from amphibolite (AMP) and granite (GR) rocks, so that the influence of the type of pedogenesis (generated by these materials) on its chemistry could be established. Umbric A1 horizons were sampled in three AMP soils (Lithic Hapludands, Andic Dystrudept), under oak, pine, and eucalyptus stands, and in three nearby GR soils (Typic Dystrudepts) under identical stands. The A1 GR horizons had a lower pH (4.2 vs 4.6), and greater aqueous soluble Al (0.40 vs 0.06 mmol kg−1) and NH4Cl-extractable Al contents (6.8 vs 2.6 cmolc kg−1) than the A1 AMP horizons. Mean values of sodium pyrophosphate extractable-Al (Alp) and ammonium oxalate extractable-Al (Alo) in the A1 AMP horizons were 12.0 and 16.2 g kg−1, respectively, and the molar ratio between Alp and sodium pyrophosphate extractable-C (Cp) ranged between 0.1 and 0.2. Mean values of Alp, Alo, and Alp/Cp in the A1 GR horizons were significantly (P< 0.05) smaller (5.2 g kg−1, 5.3 g kg−1, and 0.06, respectively). In the A1 GR horizons, we observed a lower pH (significant atP< 0.05) in the rhizosphere than in the bulk soils, a pattern which was not observed in the A1 AMP horizons. The balance of nutritive cations and anions between rhizosphere and bulk soils did not explain the patterns of pH change. Based on the relationships found among organic acids, soil pH, and Alp/Cp, we propose that, in the A1 GR horizons, where organic matter is Al-undersaturated, the increased organic acidity produced in the rhizosphere cannot be completely buffered by Al alkalinity, whereas in the AMP soils, where mineral amorphous Al coexists with Al-humus complexes, organic compounds become stabilized with reactive Al surfaces, maintaining organic acidity at low levels, even in the rhizosphere (whenever the stabilization capacity of these surfaces is not exceeded). Our results imply that both variations in organic matter content and the degree of metal saturation of the organic matter need to be considered when investigating the causes behind changes in soil pH in the rhizosphere of acid forest soils.

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