Pedogenesis in very deep autochthonous Ferralsols of the Paran?a Igneous Province (Brazil)

Ferralsols are strongly weathered soils that cover a large part of the tropics and subtropics and have been extensively studied due to their relevance for agriculture. However, the strong focus of pedological investigations on the 2 m control section of has systematically left aside records that may exist at greater depths, which are particularly relevant to understanding the formation of Ferralsols. To improve the understanding of Ferralsol pedogenesis, ten complete soil profiles were studied, from the A horizon at the soil surface to the BC horizon at the weathering front. The studied soil profiles are all located in the Paran ' a Igneous Province of southern Brazil and represent three study areas that differ in altitude and climate: Guarapuava (similar to 1000 m above sea level (a.s.l), Cfb); Cascavel (similar to 700 m a.s.l., Cfa); and Palotina (similar to 400 m a.s.l., Cfa). The absence of lithologic discontinuities, evidenced by elemental records and particle size distribution, suggests that the studied Ferralsols were formed in situ from autochthonous material. Ferralitization was evidenced by the presence of kaolinite, gibbsite, hematite, goethite, and traces of hydroxy-interlayered 2:1 clay minerals. The soil profiles from the highlands (similar to 1000 m a.s. l.) showed stronger brunification and melanization within the 2 m control section, while below the control section, rubefaction was more intense. Rubefaction was dominant in the soils from both locations with a warmer climate (similar to 400 to 700 m a.s.l.), and did not differ with depth. Micromorphology indicated aggregate densification below the control section, through a decrease in features indicative of biological activity with depth, less swelling and shrinkage cycles, and pore morphology change. Iron nodules and dense clayey nodules occurred most frequently below the control section. These pedorelict features were interpreted as formed through ferrugini-zation and pseudomorphic alteration, with a hierarchical relationship along the soil profiles and thus developed in situ. The geric properties and acric character of the Guarapuava and Cascavel Ferralsols (1000 - 700 m a.s.l.) are the result of intense ferralitization. The paradoxal eutric properties were exclusively present within the control section of the lowland profiles (Palotina: similar to 400 m a.s.l.) and suggest partial ferralitization of the rich parent material combined with more effective cycling of Ca2+ and Mg2+ carried out by roots during the Holo-cene. The detailed analyses from the topsoil to the weathering front enabled a hierarchy of pedogenetic inter-pretation on a continuum and demonstrated spatial variation of properties that often cannot be observed in the first two meters or in a single profile.

Automated summary

Ten complete soil profiles from three different areas in southern Brazil were studied to improve the understanding of Ferralsol pedogenesis. The study found that Ferralsols mainly form in the 2 m control section, but other records exist at greater depths. The detailed analysis from the topsoil to the weathering front provided a hierarchical interpretation of pedogenesis and demonstrated spatial variation of properties.