Saturday, 15 July 2006

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, and Ingrid Kögel-Knabner. Lehrstuhl für Bodenkunde (Soil Science), Department of Ecology, Technische Universität München, Am Hochanger 2, 85350 Freising-Weihenstephan, Germany

We studied 27 soil profiles developed from rhyodacite in the highlands of north-eastern Rio Grande do Sul State, Brazil. The climate is humid and temperate without marked dry periods. The vegetation comprises pastureland, not managed grassland, deciduous forest with Araucaria angustifolia, shrubland and Pinus plantations. The soils are very rich in organic matter and strongly acid, they have high clay contents, low bulk densities and high phosphate retentions. XRD analyses showed the presence of quartz, kaolinite, chlorite, and gibbsite. Extractions with pyrophosphate (p), dithionite-citrate-bicarbonate (d) and acid oxalate (o) gave 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 a high amount of Al humus complexes. The absence of allophane and imogolite was confirmed by the very low Sio and Sid concentrations.

21 soils meet the andic requirements according to the draft of the second edition of World Reference Base for Soil Resources (bulk density ≤ 0.9 kg dm-3, Alo + 0.5 Feo ≥ 20 g kg-1, phosphate retention ≥ 85 %) over at least some depth, 13 fulfil them over the required minimum thickness of an andic horizon and (the andic horizon starting within 25 cm from the soil surface) classify as Andosols. They are the first detected Andosols in South America outside the Andes. The other 6 soils fail the Alo + Feo criterion over the whole soil depth, 2 of them additionally the phosphate retention. Besides the 13 Andosols we have 12 Umbrisols and 4 Cambisols. 11 Umbrisols and the 4 Cambisols are higher weathered with high Fed and low Feo concentrations and may have developed from Andosols by crystallization of iron oxides.

15 soil profiles are under grassland and grassland-related vegetation, 12 under forest and shrubland. Forest and shrubland are more concentrated on steeper slopes and 7 soil profiles have a buried soil starting within 40 cm (buried soils were not found under grassland) and 4 others have a solum shallower than 60 cm. The upper horizons of all buried soils meet the andic requirements and, if present, form part of the andic horizon. According to pollen analyses, which indicate that the forest is not older than 3000 years, these buried soils probably developed under grassland. Generally, the upper topsoil horizons in the forest have clearly lower Feo and/or Alo concentrations, lower Feo / Fed ratios, and lower phosphate retentions than those of the grassland profiles. Contrary to 5 grassland topsoils none of them meets the andic requirements. Under forest, crystallization of iron oxides seems to be easier than under grassland.

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