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P>Latosols (Oxisols, Ferralsols) are characterized by a poor soil horizon differentiation, with a strong microgranular structure formed by microaggregates 50 to 300 mu m 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 mu m 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. Proprietes de retrait des microagregats de Latosols bresiliens: importance de la teneur en argile, de la composition mineralogique et de l'histoire hydrique. Resume Les Latosols (Oxisols, Ferralsols) sont caracterises par une faible differenciation de leurs horizons et une structure microgranulaire representee par des microagregats de 50 300 mu m de diametre. Ces microagregats sont generalement consideres comme etant tres stables et leurs proprietes de retrait comme etant tres limitees, voire absentes. L'objectif de cette etude est d'analyser les proprietes de retrait des microagregats dans l'horizon diagnostic Bw de Latosols de composition granulometrique et mineralogique variable en fonction de leur position le long d'une toposequence regionale selectionnee pour l'etude dans le Plateau Central bresilien. Nous avons mesure les quantites d'eau retenues -300 et -1500 kPa en utilisant la methode par centrifugation. La morphologie et la taille des particules elementaires ont ete etudiees en microscopie electronique transmission et l'aide de mesure de surface BET. Nous avons aussi determine la distribution de taille des pores dans les agregats deshydrates en combinant les donnees de la porosimetrie au mercure et celles issues de l'isotherme de desorption d'azote. Nos resultats montrent que les microagregats des Latosols etudies ne sont pas rigides et diminuent de volume lors de la dessiccation. Le volume poral des microagregats -300 et -1500 kPa ainsi que leur volume poral apres deshydratation sont etroitement lies la teneur en argile. En revanche, la composition mineralogique de la fraction < 2 mu m ne jouerait au plus qu'un role secondaire dans les variations du volume poral. Nos resultats ont montre que les proprietes de retrait des microagregats varient en fonction de la position des Latosols dans le paysage. Ainsi, les microagregats des Latosols situes sur les hauts plateaux de la fin du Tertiaire diminuent moins de volume lors de la dessiccation que les agregats situes sur les surfaces du Quaternaire recent et situees en contrebas dans le paysage. Une telle difference de comportement serait liee l'histoire hydrique des Latosols, ceux situes sur les surfaces les plus anciennes ayant vraisemblablement subi des contraintes hydriques plus elevees que ceux situes sur les surfaces les plus recentes.

C. R. Geosci. ISI Document Delivery No.: 761BF Times Cited: 2 Cited Reference Count: 30 Santos, Glenio Guimaraes da Silva, Euzebio Medrado Marchao, Robelio Leandro da Silveira, Pedro Marques Bruand, Ary James, Francois Becquer, Thierry CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico, Brazil); Embrapa Cerrados-IRD [0203205] This research is part of the project Embrapa Cerrados-IRD, No. 0203205 (Mapping of the Biome Cerrado Landscape and Functioning of Representative Soils). This work was supported by a grant awarded to G.G. Santos by the CNPq (Conselho Nacional de Desenvolvimento Cientifico e Tecnologico, Brazil) Elsevier france-editions scientifiques medicales elsevier Paris; International audience; Among the various soil indicators established in order to discuss physical properties of soils is the S-index, derived from the slope of the soil water retention curve at its inflection point, used by a number of authors. In this publication we discuss the value of the slope at the inflection point of the soil water retention curve according to the independent variable used to plot it. We show that a representation of the water content according to the arithmetic rather than logarithmic expression of the suction for the S-index yields a different result for the soil selected. More generally, our results show that examining the physical properties of soil using a water retention curve plotted with an arithmetic expression of suction offers greater potential than when plotted with its natural or decimal logarithm as is often found in the literature. Parmi les différents indicateurs qui ont été proposés pour rendre compte des propriétés physiques du sol, l'indice S, qui correspond à la pente de la courbe de rétention en eau du sol à son point d'inflexion, a été largement utilisé. Dans cet article, nous discutons de la valeur de la pente au point d'inflexion de la courbe de rétention en eau du sol, en fonction de la variable indépendante qui est utilisée pour le déterminer. Nous montrons que la représentation de la teneur en eau en fonction de l'expression arithmétique de la succion au lieu de son expression logarithme, comme pour l'indice S, conduit à un résultat différent pour le sol sélectionné. Plus généralement, nos résultats montrent qu'une discussion des propriétés physiques du sol, en utilisant une représentation de la courbe de rétention d'eau en fonction de l'expression arithmétique de la succion, offre plus de possibilités que l'expression logarithmique naturelle ou décimale qui a été largement utilisée jusqu'alors.

International audience; Brazilian Latosols are characterized by low activity clay, little horizon differentiation, a weak macrostructure and a strong microgranular structure. The development of the latter that was reported as being possibly related to the fine material mineralogy, location in the landscape, parental material and biological activity is still under discussion. The aim of this study is to discuss the origin of the microgranular structure of Latosols located in the Brazilian Central Plateau. Ten Latosols (L) developed in different parent materials were selected along an approximately 350-km long regional toposequence across the South American Surface (SAS) (L1 to L4) and Velhas Surface (VS) (L5 to L10). The structure of the Latosols was studied in the field and samples of the diagnostic Bw horizons were collected for laboratory analyses. Basic soil characterization was performed on the

International audience; This study aims at comparing the centrifuge and pressure plate methods when appropriate duration of run is adopted. Samples collected in tropical soils located in Brazil along a 10-km local hydrosequence across the Cerrado-Amazonia transition (set 1) and along a 350-km regional toposequence across the Cerrado region (set 2) were selected to compare and discuss statistically the similarity of the soil-water retention recorded by using the pressure plate and centrifuge methods. The results showed good agreement (R2 = 0.99) for the 1:1 comparison of measured point-wise soil-water content values (set 1) as well as for the fitted soil-water content curves by van Genuchten model using data points obtained with the two methods (set 2). Thus, the centrifuge method should be considered as an appropriate method for determining soil-water retention properties not only for the similar results with the pressure plate method but also for being much less time consuming.

International audience; The mineralogy of the Latosols of the Brazilian Central Plateau remains under discussion in the absence of a clear relationship with their age according to their geomorphic location. The aim of this study was thus to clarify the origin of the kaolinite and gibbsite content variation by studying a regional toposequence and using data from the literature. Chemical composition and soil color were used to discuss mineralogy. The mineralogy of the clay fraction was also investigated using X-ray diagrams. Our results showed that the large variation of kaolinite and gibbsite content can be explained by taking into account both their local and regional location, the variation of the hematite and goethite content remaining limited. The model that is proposed to explain such variation combines a regional component, which is mainly associated to the age of the geomorphic surface and a local component which is mainly associated to the hydraulic conditions along the toposequence. La minéralogie des latosols du Plateau Central brésilien est discutée en l'absence de relation clairement établie avec leur âge qui est en fonction de leur position géomorphologique. L'objectif de cette étude est par conséquent de clarifier l'origine de la variation de teneur en kaolinite et gibbsite en étudiant une toposéquence régionale et les données de la littérature. La minéralogie a été discutée à partir de la composition chimique et de la couleur du sol. Elle a aussi été discutée à l'aide des données de la diffraction des rayons X. Les résultats montrent que la variation élevée de la proportion de kaolinite et de gibbsite des latosols peut être expliquée en prenant en compte à la fois leur localisation régionale et locale. Le modèle proposé combine en effet une composante régionale qui est principalement liée à l'âge de la surface géomorphologique et une composante locale qui est principalement liée aux conditions hydriques le long de la toposéquence.

International audience; The Brazilian Central Plateau is constituted of the South American Surface and Velhas Surface geomorphologic surfaces and represents 24% of the Brazilian territory. Latosols cover about 40% of these Surfaces. Kaolinite, gibbsite, goethite and hematite were recognized in different proportions in the clay fraction of Latosols by many authors. The objective of this work was to analyse the kaolinite cristalinity and to discuss its variability according to the Latosol location on the geomorphic Surfaces of the Brazilian Central Plateau.

International audience; The Brazilian Central Plateau Latosols occurs on Tertiary South American and Velhas planning surfaces. The purpose of this work was to determine activation energy of kaolinite dehydroxylation and its genesis relationships of the Brazilian Central Plateau Latosols.

International audience; The Brazilian Central Plateau covers about 40% of the Cerrados Biome and represents 24% of the Brazilian territory. The Latosols that correspond to about 40 % of the surface area of the Central Plateau are characterized by a poor horizonation, a weak macrostructure and a strong development of the fine granular structure composed of sub-rounded microaggregates 50 to 300 µm in size. In this study, we analyzed the hydraulic properties of a set of Latosols varying according to their clay content and mineralogy with respect to their location along a regional topossequence across the Brazilian Central Plateau. Ten Latosols (L) were selected on the South American Surface (L1 to L4) and Velhas Surface (L5 to L10) and we studied the properties of their diagnostic horizon (Bw). We measured their bulk density and particle density, and the soil-water retention properties at 1, 6, 10, 33, 300, and 1500 kPa by using the centrifugation method. We also determined the saturated hydraulic conductivity in the field using the Guelph permeameter procedure. Results showed that the total pore volume (Vp) ranged from 0.460 to 0.819 cm3 g-1 and 58.2 % of the variance was explained for by the clay content. According to Balbino et al. (2002), Vp was divided into a volume of intra-microaggregates pores (Vintra) and inter-microaggregates pores (Vinter). Results showed that Vintra ranged from 0.090 to 0.234 cm3 g-1 and Vinter from 0.305 to 0.585 cm3 g-1. Results showed also that Vp explained a proportion of the variance of the water retained that decreased with the water potential. On the other hand, the clay content explained a proportion of that variance that increases when the water potential decreased. The great proportion of variance (90.7 %) explained for by the clay content alone at 1500 kPa showed that there is little variability that can be attributed to clay mineralogy variation. The saturated hydraulic conductivity (Ks) was related to an effective porosity (Φe) defined as the volume proportion of pore with equivalent diameter > 300 µm. Finally, our results showed that water retention properties and saturated hydraulic conductivity varied mainly according to the clay content and development of large pores without any close link with the mineralogy of the clay fraction.

The Brazilian Central Plateau is constituted of the two main geomorphic surfaces: the South American surface and the Velhas surface. These surfaces were developed during the Cretaceous Superior and Tertiary and are covered by deeply weathered Latosols. The South American surface is the oldest and occupies the highest position in the landscape (950 to 1,200 m altitude) with smoothly convex plane portions. The Velhas surface is composed by irregular and slightly sloping planes and occupies in the landscape (750 to 950 m altitude). It is connected to the South American surface by areas of steep hillsides.

Latosols are deeply weathered soils characteristic of the moist tropics. Many Latosols have kaolinitic horizons overlying more gibbsitic ones (Lucas, 1989), for which there has been no satisfactory pedological explanation and why they have often been considered as a sedimentary succession. Several hypotheses to explain this phenomenon were expressed: biogeochemical recycling of Si by the forest (Lucas et al., 1993) or the activity of termites (Volland-Tuduri, 2005). The objective of this study is to model the development of Latosols of the Brazilian Central Plateau considering the respective effect of parent material, time or biological activity. This plateau is constituted of the two main geomorphic surfaces: the South American surface (Suface I) and the Velhas surface (Surface II). In the Central Plateau, the Latosols can be identified as Red Latosols (~28%), Yellow Red Latosols (~10%) and Yellow Latosols (~2%), (Reatto, 1998).

The Cerrados Biome is one of the major regions of Brazil with 24% of the Brazilian territory (204.106 ha). It is mainly located in the center of Brazil and corresponds to the whole Central Plateau. About 49% of the soils are Ferralsols and approximately 79.106 ha of these soils are dedicated to agriculture. The main characteristics of Ferralsols are a poor horizonation, a weak development of the macrostructure, and a strong submillimetric granular microstructure.

Ferralsols have a ferralic horizon at some depth between 30 and 200 cm that results from long and intense weathering. Their clay fraction is usually mainly low-activity clay consisting of kaolinite with hematite, goethite and gibbsite in different proportions. Ferralsols show little or no horizonation, and their macrostructure is absent to moderate. On the other hand, they have typically a strong microstructure consisting of microaggregates < 1 mm in size. Because of the lack or small development of macrostructure, porosity of Ferralsols is closely related to the development of microstructure and the assemblage of elementary particles within the microaggregates with a small contribution of large pores resulting from root development and macrofaunal activity. Their physical properties are then closely related to the development of this microstructure. However, there is still no model in the literature that predicts changes of microstructure of these soils using easily accessible soil properties when land-use is modified. The objective of this work was to relate microstructure development to the bulk density (Db) in Ferralsols and then to make possible the use of Db as an indicator of microstructure development.

The physical properties of Brazilian Oxisols are closely related to the development of their microstructure, which typically consists of stable microaggregates >1 mm. There is no model available to predict changes in microstructure in Oxisols. The objective of this work was to relate the proportion of microaggregates to the bulk density (D b ) in the soil studied. Five sites of a typic Haplustox under native vegetation (two sites) and pasture (three sites) were sampled. Soil bulk density, sand, silt, and clay content and aggregate-size distribution were measured from the surface to 1.6 m deep in increments of 0.1 m. Thin sections were prepared from undisturbed samples collected in duplicate from 0- to 0.1-, 0.3- to 0.4-, 0.8- to 0.9-, and 1.5- to 1.6-m depths, and backscattered electron scanning images (BESI) were generated. Clay content ranged from 672 to 798 g kg - 1 and bulk density between 0.87 and 1.18 g cm among the 80 samples studied. Bulk density was poorly correlated with clay content (R 2 = 0.358) and at any depth was not significantly smaller under native vegetation than under pasture. Visual assessment of BESI revealed that soil material corresponded to either microaggregates (

Among microaggregated soils, there are Ferralsols that show little or no distinct horizonation. Their macrostructure is weak to moderate and they have typically a strong microstructure. In most Brazilian clayey Ferralsols, physical properties are closely related to the development of microstructure that consists of subrounded microaggregates 50 to 500 µm in diameter.