In Brazilian Amazonia there are 26 million hectares of degraded pastures. One of the causes of degradation is assumed to be alterations in soil structure that is partly determined by a reduction in soil macrofauna diversity. The objective of this case study was to identify the consequences of altered macrofaunal diversity on soil structure and hydrological processes. After forest clearing for pasture establishment, the diversity of soil macroinvertebrates decreased strongly. Tensiometric measurements showed the formation of a superficial groundwater table in a 4-year-old pasture (P4), and a deeper groundwater table in an abandoned pasture (Pa), that was an exceptional situation. The forest soil, in contrast, was almost never water-saturated due to its microaggregate structure. Once the pasture was abandoned, the soil macrofauna was dominated by a single earthworm species, Pontoscolex corethrurus, which may modify, in the course of a few years, the morphology and hydrological functioning of soils. By placing very wet castings on the pasture soil surface, P. corethrurus can form a continuous layer that is relatively impermeable to air and water. Although initially limited to the surface layer, the episodic conditions of water saturation and anoxia may also occur in deeper horizons, favoring the development of hydromorphic conditions, even though the surface soil structure may be regenerating due to the activity of other macrofaunal groups such as termites, after reduction of P. corethrurus populations. © 2003 Elsevier B.V. All rights reserved.
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