A mosaic of nonallophanic Andosols, Umbrisols and Cambisols on rhyodacite in the southern Brazilian highlands Alexander Dümig, Peter Schad ⁎ , Manmath Kohok, Patrick Beyerlein, Wolfgang Schwimmer, Ingrid Kögel-Knabner Lehrstuhl für Bodenkunde, Department für Ökologie, Wissenschaftszentrum Weihenstephan für Ernährung, Landnutzung und Umwelt, Technische Universität München, D-85350 Freising-Weihenstephan, Germany Received 26 May 2006; received in revised form 24 January 2007; accepted 29 January 2008 Available online 17 March 2008 Abstract Soils in the highlands of north-eastern Rio Grande do Sul (Brazil) show dark-coloured topsoils which are light and very rich in organic matter. They are distributed in a mosaic of grassland, forest with Araucaria angustifolia and several anthropogenic vegetation forms. The climate is humid and temperate without marked dry periods. We characterized and classified 27 soils to investigate the relationships between present and past vegetation pattern and soil properties. All soils are strongly acidic with high clay contents, low bulk densities and most of them show high phosphate retentions. X-ray diffraction indicates the presence of quartz, kaolinite, secondary chlorite and gibbsite. Selective extractions with pyrophosphate (p), dithionite–citrate–bicarbonate (d) and acid oxalate (o) give in most horizons the following relationships: Fe d N Fe p N Fe o and Al p N Al d N Al o . Despite the possible Al release from gibbsite, we attribute the high Al p concentrations mainly to high amounts of Al–humus complexes. The absence of allophane and imogolite was confirmed by the very low Si o and Si d concentrations. 21 soils have andic properties according to World Reference Base for Soil Resources (WRB) [IUSS Working Group WRB, 2006. World Reference Base for Soil Resources 2006. World Soil Resources Reports, 103. FAO, Rome.] at least for some depth range, 15 of them fulfil the minimum thickness and depth requirements of Andosols. They belong to the aluandic type and are the first detected Andosols in South America outside the areas of recent volcanism in the Andes. The other soils are Umbrisols (11) and Cambisols (1). In grassland soils, andic properties start at or near the soil surface, in forest soils they are only present in subsurface layers, especially in buried A horizons. The latter differ from their actual topsoils by some characteristics which they have in common with the grassland soils like deep black colours and low melanic indexes. It is known from pollen analyses that the Araucaria forests are invading the grasslands since about 1000 years. In this environment a grass vegetation seems to favour formation and permanence of Andosols whereas a change into the forest starts a process of losing andic properties. Most soils without andic properties or with andic properties of insufficient thickness only fail the required minimum concentrations of Al o and Fe o . On the other hand, they all show similar concentrations of Al d and Fe d . Therefore we conclude that the loss of andic properties with forest proliferation is caused by crystallization of Al and Fe oxides which seems to be easier under forest. © 2008 Elsevier B.V. All rights reserved. Keywords: Andic properties; Pedogenesis; Soil classification; Organo-metallic complexes; Grassland; Vegetation effects 1. Introduction Andosols according to the World Reference Base for Soil Resources (IUSS Working Group WRB, 2006) have andic or vitric properties. A prerequisite for vitric properties is a minimum content of volcanic glasses. Andic properties show low bulk densities, large variable charge and high phosphate retentions which are linked mainly to the accumulation of various reactive forms of Al in the solid phase. Andic properties are distinguished between allophanic, where Al is present mainly in short range order minerals like allophane or imogolite and nonallophanic, where Al in organic complexes predominates (Shoji and Fujiwara, 1984; Dalhgren and Saigusa, 1994). Both types commonly show considerable amounts of ferrihydrite and a Available online at www.sciencedirect.com Geoderma 145 (2008) 158 – 173 www.elsevier.com/locate/geoderma DOI of original article: 10.1016/j.geoderma.2007.06.005. ⁎ Corresponding author. Tel.: +49 8161714735; fax: +49 8161714466. E-mail address: schad@wzw.tum.de (P. Schad). 0016-7061/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2008.01.013