Accumulation of organo-metallic complexes in laterites and the formation of Aluandic Andosols in the Nilgiri Hills (southern India): similarities and differences with Umbric Podzols

Abstract : We have investigated the speciation and distribution of iron (Fe) and aluminium (Al) between minerals and organic species in A and B horizons of two Aluandic Andosols with X-ray diffraction, thermal analyses, visible diffuse reflectance and Fourier transform infrared (FTIR) spectroscopies, together with selective and total chemical extractions. The two Aluandic Andosols of the Nilgiri Highlands (south India) have formed at the expense of intensively weathered lateritic formations of the Eocene. Data revealed that Al and Fe were predominantly stored in end-weathering products of laterites, mainly as gibbsite and Fe (hydr)oxides in B horizons of the Aluandic Andosols. These secondary minerals are gradually replaced by organo-metallic complexes in the topsoil A horizons exhibiting andic properties. We then indicate that formation of the organo-metallic complexes results from weathering of the dominant crystalline Al- and Fe-(hydr)oxides mediated by the organic ligands and complexation of the polyvalent metals following the accumulation of organic matter. Such weathering and complexation mechanisms are therefore similar to those recently ascribed to deferralitization and the formation of freely drained Umbric Podzols (Humus-Podzols) in the upper Amazon Basin. In the present case, large supplies of both Al- and Fe-bearing minerals provide large metal:carbon ratios that prevent the mobility of the organo-metallic complexes and induce the formation of Aluandic Andosols rather than Podzols.