Mass proportion of microaggregates and bulk density in a Brazilian clayey Oxisol

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 (phi < 0.84). Finally, our results showed that phi < 0.84 varied with D(b). Linear regression coefficients were calculated for the relationship between phi < 0.84 and the reciprocal of bulk density (1/D(b)) [phi < 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.