Shrinkage of microaggregates in Brazilian Latosols during drying: significance of the clay content, mineralogy and hydric stress history

Abstract : Latosols are characterized by a poor differentiation of the horizons, a weak macrostructure and a strong submillimetric granular structure resulting in microaggregates 50 to 300 µm in size. The shrinkage properties of these microaggregates that are generally considered as very weak or absent are still under discussion. The objective of our study is the analyze the shrinkage properties of microaggregates in diagnostic Bw horizons collected in a set of Latosols varying in their particle size distribution and mineralogical composition according to their location in a regional toposequence across the Brazilian Central Plateau. We measured the water retention properties at 300 and 1500 kPa by using the centrifugation method. The morphology and size of the elementary particles was studied by using transmission electron microscopy and determining the specific surface area (SSA) with the BET method. We determined also the pore size distribution of the microaggregates by combining mercury intrusion and nitrogen desorption isotherm. Our results showed that the microaggregates of the Latosols studied are not rigid but shrink during drying. The pore volume of the microaggregates at –300 and –1500 kPa as well as the pore volume of the dried microaggregates are closely related to the clay content. The SSA is also closely related to the clay content. Consequently, the mineralogy of the <2 µm would play at the most a marginal role in the properties studied. Finally, our results showed that the shrinkage of the microaggregates between –300 kPa and the shrinkage limit and between –1500 kPa and the shrinkage limit varies with the location of the Latosols in the Landscape. Thus, the microaggregates of the Latosols located on the SAS shrink less than those of the Latosols located on the VS. This difference might be related to a difference of hydric stress history of the Latosols.