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Soil Physics

Mass Proportion of Microaggregates and Bulk Density in a Brazilian Clayey Oxisol

^a Institut des Sciences de la Terre d'Orléans (ISTO), UMR 6113 CNRS-UO, Université d'Orléans, Géosciences, BP 6759, 45067 Orléans Cedex 2, France
^b IRD, BP 64501, 34394 Montpellier Cedex 5, France
^c EMBRAPA Arroz e Feijão, C.P. 179-CEP 75.375-000 Santo Antônio de Goiás–GO, Brazil
^d EMBRAPA Cerrados, C.P. 08223, 73301-970 Planaltina-DF, Brazil

^* Corresponding author (Ary.Bruand{at}univ-orleans.fr)

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^–3 among the 80 samples studied. Bulk density was poorly correlated with clay content (R² = 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 (<0.1 mm) in loose arrangement or to much larger aggregates. Quantification of BESI from the deepest sampling depth of all soils showed that 96.2 and 95.7% of microaggregates were <0.8 mm with 73.2 and 95.7% between 0.1 and 0.5 mm under native vegetation and pasture, respectively. The mass proportion of microaggregates can be estimated using the <0.84-mm soil material that is obtained by dry sieving ( < 0.84). Finally, our results showed that < 0.84 varied with D_b. Linear regression coefficients were calculated for the relationship between < 0.84 and the reciprocal of bulk density (1/D_b) [ < 0.84 = 1.97 (1/D_b) – 1.52, R² = 0.82], assuming no interaction between microaggregates and macroaggregates, the porosity of these two structural types was estimated as 0.71 and 0.51, respectively.

Abbreviations: BESI, backscattered electron scanning images