Consequences of transformation of native Cerrado in pasture land on the porosity of a clayey-sand red latosol

In the Brazilian savannah (Cerrados area), about 60 % of the surface area is used as pasture land and 30 % of the latter is cultivated pasture. Many studies have reported that pastures degrade quickly and degradation would be more intense when the soil was tilled prior pasture sowing. Pasture degradation (decrease in the pasture quality and yield, weed invasion) would be partly a consequence of the soil structure evolution (Balbino & Di Stefano, 1999). Origin of pasture degradation is still under discussion and analysis of the literature showed that there is a lack of quantitative data. The aim of this study is to perform a detailed analysis of the porosity in a low productivity cultivated pasture and to compare the results with the porosity of a similar soil still under native vegetation (Cerradão, i.e. Scierophylous forest)

Change in the hydraulic properties of a Brazilian clay Ferralsol on clearing for pasture.

32 ref. doi: 10.1016/j.geoderma.2003.08.017Ferralsols under native vegetation have a weak to moderate macrostructure and a well-developed microstructure corresponding to subrounded microaggregates that are usually 80 to 300 μm in size. The aim of this study was to analyze how the hydraulic properties of a clay Ferralsol were affected by a change of structure when the native vegetation is cleared for pasture. We studied the macrostructure in the field and microstructure in scanning electron microscopy. The water retention properties were determined by using pressure cell equipment. We determined the saturated hydraulic conductivity, Ks, by applying a constant hydraulic head to saturated core samples, and the unsaturated hydraulic conductivity, K(Ψ), by applying the evaporation method to undisturbed core samples. Results showed a significant decrease in the water retained at −1 and −10 hPa from 0- to 40-cm-depth when the native vegetation is cleared for pasture. That decrease in the water retained was related to a smaller development of microaggregation and greater proportion of microaggregates in close packing. For smaller water potential, there was no difference of water retained at every depth between native vegetation and pasture. Pedotransfer functions established earlier for Brazilian Ferralsols and using clay content as single predictor gave pretty good results but the precision of the estimation decreased when the water potential increased. This decrease in the precision was related to the lack of predictor taking structure into account. Ks and K(Ψ) showed an upward trend with depth under native vegetation and pasture. Except at 0–7-cm depth between the Brachiaria clumps in the pasture where smaller Ks and K(Ψ) than at the other depth was recorded whatever land use, we did not record any significant difference of Ks and K(Ψ) at every depth between native vegetation and pasture. The upward trend shown by the hydraulic conductivity with depth was related to the increase in the development of microaggregation with depth

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 smaller than 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 (Db) 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 0-0.1 m, 0.3-0.4 m, 0.8-0.9 m and 1.5-1.6 m depth, 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. Db 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 Db. Linear regression coefficients were calculated for the relationship between &<0.84 and the reciprocal of bulk density (1/ Db) (&<0.84 = 1.97 (1/ Db) – 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

Modelling of bulk density as related to aggregate size distribution in clayey Ferralsols

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

Modelling Bulk Density According to Structure Development: Toward an Indicator of Microstructure Development in Ferralsols.

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

Changes in porosity and microagregation in clayey Ferralsols of the Brazilian Cerrado on clearing for pasture.

Ferralsols have a macrostructure that is weak to moderate and a strong microstructure consisting of near spherical microaggregates. We have studied the evolution of the structure under changing land use in two Ferralsols under a native Cerrado and under pasture sowed after recent clearing by measuring bulk density. We studied the microaggregates' characteristics and resulting porosity using scanning electron microscopy and mercury porosimetry, respectively. Microaggregates 50–300 μm in diameter are attributed to termites. They were in a much closer arrangement from the surface to 1 m depth under pasture than under Cerrado. Larger microaggregates, 500–1000 μm in diameter, that are closely packed, resulted from earthworm activity. They were more numerous under pasture than under Cerrado. Thus within a few years, clearing and grazing have caused a dramatic decrease in microstructure that is one of the rare favourable characteristics of Ferralsols for agriculture. This evolution that affects the subsoil to 80–90 cm cannot be attributed to compaction by mechanical deforestation alone but seems to result from a change in the faunal activity in the soil. Finally, the packing of the clay particles within the microaggregates was similar whatever the type of microaggregate and land use. Our results accord with earlier studies showing that the porosity resulting from clay packing varies little in Ferralsols in which the fine material is mainly kaolinite