Changes in porosity and microagregation in clayey Ferralsols of the Brazilian Cerrado on clearing for pasture.

Abstract : Ferralsols have a macrostructure that is weak to moderate and a strong microstructure consisting of near spherical microaggregates. We have studied the evolution of the structure under changing land use in two Ferralsols under a native Cerrado and under pasture sowed after recent clearing by measuring bulk density. We studied the microaggregates' characteristics and resulting porosity using scanning electron microscopy and mercury porosimetry, respectively. Microaggregates 50–300 μm in diameter are attributed to termites. They were in a much closer arrangement from the surface to 1 m depth under pasture than under Cerrado. Larger microaggregates, 500–1000 μm in diameter, that are closely packed, resulted from earthworm activity. They were more numerous under pasture than under Cerrado. Thus within a few years, clearing and grazing have caused a dramatic decrease in microstructure that is one of the rare favourable characteristics of Ferralsols for agriculture. This evolution that affects the subsoil to 80–90 cm cannot be attributed to compaction by mechanical deforestation alone but seems to result from a change in the faunal activity in the soil. Finally, the packing of the clay particles within the microaggregates was similar whatever the type of microaggregate and land use. Our results accord with earlier studies showing that the porosity resulting from clay packing varies little in Ferralsols in which the fine material is mainly kaolinite.