Shrinkage of microaggregates in Brazilian Latosols during drying: significance of the clay content, mineralogy and hydric stress history
2009
Reatto, A. | Bruand, A. | Silva, E.M. | Guégan, R. | Cousin, I. | Brossard, M. | Martins, E.S.
Latosols (Oxisols, Ferralsols) are characterized by a poor soil horizon differentiation, with a strong microgranular structure formed by microaggregates 50 to 300 μm in size. This microgranular structure is generally considered to be very stable and the shrinkage properties of microaggregates as poor or absent. The objective of our study was to analyse the shrinkage properties of microaggregates in diagnostic Bw horizons collected in Latosols with various particle size distributions and mineralogical compositions depending on their location in a regional toposequence across the Brazilian central plateau. We measured the water retention properties at -300 and -1500 kPa with a centrifugation method. The morphology and size of elementary particles were examined by transmission electron microscopy; specific surface area (SSA) was determined by nitrogen adsorption. We also determined the pore size distribution of microaggregates by the combined use of mercury intrusion and the nitrogen desorption isotherm. Our results show that microaggregates of the Latosols studied were not rigid and shrank during drying. The pore volume of the -300 and -1500 kPa-treated samples, that of the dried microaggregates and the SSA were closely related to the clay content. The mineralogy of particles <2 μm played a secondary role in the properties studied. Our results show that microaggregate shrinkage at -300 kPa and -1500 kPa varied with the location of the Latosols in the landscape. Thus, microaggregates of Latosols located on the upper highland surface of Late Tertiary shrank less than those of the lowland surface of Late Quaternary, and this difference was attributed to their different hydric stress histories.
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