Brazilian latosols and their B horizon microstructure as long-term biotic constructs

Abstract

This paper discusses micropedological evidence for the biological formation of microstructure in tropical soils, placing emphasis on latosol (oxisol) formation promoted by termite activity. The microstructure of selected latosols from Brazil was investigated using standard chemical and physical methods, optical microscopy, and high resolution scanning electron microscopy coupled with energy-dispersive X-ray analysis to provide microchemical analysis of discrete microaggregates in thin section. The results showed that, in spite of great variability of parent materials, the B horizon of these soils displayed uniform microgranular structure, with little lithodependence and no apparent relationship to particle-size. Lithorelicts of oval pellets of 100–1000 mm diameter, similar to those in the B horizon, were observed in the upper parts of the C horizon (saprolite). In addition, microparticles of charcoal (<50 m) were found in the inner microaggregates. The basic skeleton of the inner microaggregates was formed of quartz grains, generally smaller than 100 mm diameter. In contrast, the quartz grains of the soil skeleton ranged between 30 and 5000 mm. These microaggregates were not distinguishable from those built by termites on shallowsaprolite. I present a general model of latosol genesis, which considers the close interdependence between the advent of angiosperm-dominated landscapes, neotectonics, and the synchronous appearance of termites and latosols on tropical land, dating back to late Cretaceous/early Tertiary times. The microaggregation of latosols is viewed as a long-term strategy favouring physical characteristics to counteract the irreversible trend of increasing nutrient losses. The degree of microaggregation appears related to high gibbsite and Fe-oxide concentrations, although these minerals are not responsible for microaggregate formation. However, it is postulated that gibbsite and Fe-oxides are associated only with the persistence of microaggregates in tropical soils. Because of a tendency to form face-to-face structures, kaolinite does not favour the microaggregation phenomenon, and hence, highly kaolinitic latosols display coalesced aggregates.