A mosaic of nonallophanic Andosols, Umbrisols and Cambisols on rhyodacite in the southern Brazilian highlands

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: Fed > Fep > Feo and Alp > Ald > Alo. Despite the possible Al release from gibbsite, we attribute the high Alp concentrations mainly to high amounts of Al–humus complexes. The absence of allophane and imogolite was confirmed by the very low Sio and Sid 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 Alo and Feo. On the other hand, they all show similar concentrations of Ald and Fed. 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.