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

Abstract

Ferralsols 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, K s, 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. K s 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 K s and K( Ψ) than at the other depth was recorded whatever land use, we did not record any significant difference of K s 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.