Soil formation and properties along a sedimentary lithosequence in the ecotonal Cerrados of Mato Grosso, Brazil

Abstract

Despite numerous studies on the Cerrado savannas core areas in Brazil, soils located in its transition with Amazonia and other biomes are still poorly known, although these areas are currently undergoing massive agricultural expansion. Here, we describe a lithosequence of three soils developed from sedimentary rocks along a cuesta in Mato Grosso, namely a clayey Ferralsol (65–70% clay), an Arenosol and an Alisol (both 77–95% sand). All soils were nutrient-poor and acidic, but exchangeable aluminum was generally low except for the very high values in B horizons of the Alisol. Clay mineralogy varied widely: the Ferralsol is dominated by gibbsite and has highly crystalline kaolinite, Fe and Ti oxides. Conversely, the Arenosol has poorly crystalline kaolinite and no gibbsite, and in the Alisol clays were mostly an interstratified complex between poorly-crystalline kaolinite and a 2:1-type mineral, especially in B horizons, rich in exchangeable Al and gibbsite. Coarse biotite and fine-grained micas within the Alisol matrix were visible in thin sections, and were the likely source of the interstratified phase and the excess Al in the exchange complex. Clay coatings indicative of illuviation were evident, and also Mn oxides and Fe nodules apparently disintegrating, suggesting a limited drainage in Bt horizons of this Alisol. Although the three soils have in common their development from pre-weathered sedimentary parent materials, warm humid climate and similar native vegetation, the lithosequence showed contrasting soil formative processes, composition and land surface age. These contrasts suggest a considerable pedological diversity instead of the often assumed, homogeneously extremely-weathered soil, which can have implications for soil genesis, management and ecological functions in the humid tropics.