The role of macrofauna in the transformation and reversibility of soil structure of an oxisol in the process of forest to pasture conversion

In a central Amazonian pasture, a single earthworm species, Pontoscolex corethrurus, becomes very abundant (400 ind. m(-2)) after forest clearing. Its casts form a compact, continuous and impermeable crust with a thickness of 20 cm. To analyze the structural modifications, we established a field experiment in which soil blocks from the forest were implanted in the pasture, and soil blocks from the pasture were implanted in the forest. The objectives were (1) to verify the formation of the compact crust at the soil surface in pasture environment, (2) to evaluate the time necessary for the formation and the destruction of this crust, and (3) to find out if the crust formation was a reversible process. We used quantitative morphology to identify the biogenic structures formed by different fauna groups and to quantify the modifications in the solid phase as well as the resulting porosity. For the soil depth 0-5 cm, the measured porosity was 48% in the forest and 16% in the pasture. After 1 year, the blocks of forest soil installed in the pasture presented a porosity of 26%, and the blocks of pasture soil installed in the forest presented a porosity of 34%. There were significant differences between the control blocks and the exchanged blocks. The results demonstrate that the processes of formation and destruction of the biogenic structures are reversible. Approximately 1 year is necessary to re-establish the equilibrium between the exchanged blocks and the control blocks. This experiment illustrates the compacting effect of P. colethrurus. In addition, the small millimetric pores, which are formed by termites in the blocks of pasture soil implanted in the forest, show the decompacting effect of certain termite groups. (C) 2001 Elsevier Science B.V. All rights reserved.