Genesis of the sombric horizon in ultisols (red argisols) in southern Santa Catarina, Brazil

Jaime Antonio De Almeida,Denis Cesar Cararo,Antônio Ayrton Auzani Uberti

doi:10.1590/s0100-06832009000200018

Abstract

Dark subsurface horizons, with properties similar to the sombric horizon characterized by the USA Soil Taxonomy, are frequent in Southern Brazil. The genesis of this horizon is controversial and poorly understood. This study aimed to describe the occurrence of sombric-like horizons in Ultisols in the South of Santa Catarina State, at low altitudes, and suggest possible processes of humus transference, accumulation and persistence in these horizons. Physical, chemical and mineralogical properties of four Ultisols were evaluated; three were sampled in a toposequence, and another representative one in an isolated profile (RSP). The dark subsurface horizons coincide with the AB and BA transitional genetic horizons; they are acid, low in base saturation, and have a similar clay mineralogy in all horizons. Very high amounts of Fe and Al extracted by ammonium oxalate and sodium pyrophosphate solution as well as maximum Al extracted by CuCl2 solution were observed in these dark subsurface horizons, indicating a possible migration of these elements in the form of organometallic complexes. The contents of Al plus ½ Fe extracted from the RSP soil horizons with ammonium oxalate indicated spodic materials in the sombric-like horizon, although the soil morphology was not compatible with Spodosols. Maximum contents of fine clay were also found in the sombric-like horizon, suggesting Fe and Al migration as clay-humic substances. However, the hypothesis that sombric-like horizons in these soils are a relict feature of a grass paleovegetation, different from the current dense seasonal forest, should not be discarded but investigated in further studies.

Highlights

In the South of Santa Catarina State, in the surroundings of the cities of Criciúma and Içara, there is a large area of Ultisols with a peculiar feature; dark subsurface horizons are interlayered between lighter A and red B horizons
Sombric horizons were first identified by Belgian pedologists who worked in the Congo, nowadays Zaire, where they were found a number of soil types such as Ultisols, Alfisols, Inceptisols and Oxisols
In the three profiles a pronounced clay increment was observed in the deeper layers, with maximum values in the horizons Bt1 (P1) and Bt2 (P2 and P3); the texture gradient in these soils is much lower than in the representative profile (RP) (Table 2) and the clay contents higher than in the surface horizons

Summary

Introduction

In the South of Santa Catarina State, in the surroundings of the cities of Criciúma and Içara, there is a large area of Ultisols with a peculiar feature; dark subsurface horizons are interlayered between lighter A and red B horizons. The position of these horizons is coincident with that of transitional AB and, or BA horizons. Sombric horizons were first identified by Belgian pedologists who worked in the Congo, nowadays Zaire, where they were found a number of soil types such as Ultisols, Alfisols, Inceptisols and Oxisols Their occurrence seems to be restricted to cold and mountainous intertropical regions, at altitudes between 1.500 and 2.000 m asl

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