Abstract

In western Burkina Faso, in the Bondoukuy cotton zone, on silty-clay soils, the rarefaction of the perennial grass Andropogon gayanus, characteristic of old fallows, is noticed. This grass is known to restore the structural stability of the soil rather quickly, due to a stimulation of biological activity, while the storage of organic matter in the soil remains slow. The biological aspect has been prospected through the study of invertebrate populations in 3 sites, all of which present 4 situations: 30-year-old fallows, 5-year-old natural fallows, and 5-year-old A. gayanus improved fallows and 10-year-old permanent crop fields.

The results show that invertebrate populations are very diverse. A total of 172 morphotypes were recorded, including 115 species of insects (24 species of social insects), 31 species of Chelicerata, 8 Myriapoda and 5 earthworms. In the fields, earthworms and Diplopoda populations are less important than in old fallows. On the other hand, the fields have the highest species diversity, 50 species on average. The most abundant population is found in natural fallows, with a density of 835 individuals/m² and 43 species represented. Improved fallows with A. gayanus have a lower stand density and species richness than natural fallows (less than 50% and 43 species). In old fallows, stands are stable with a richness limited to 30 species, while earthworms and myriapods have significantly reduced densities.

Soil invertebrate communities thus recover rapidly after crop abandonment and are fully active during the most intense phase of recovery in the first 5 years. Trees do not appear to have a significant effect on the conservation and stimulation of macrofauna except for Coleoptera.

Our results show that the cropping system adopted in the region allows for minimal conservation of soil macrofauna and that the macrofauna recovers rapidly during the fallow. Improved fallows with A. gayanus differ from natural fallows in limiting Termite density, while old fallows and fields are of comparable density.

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