Regulation of soil structure by geophagous earthworm activities in humid savannas of Côte dʹIvoire

The role of endogeic earthworms in the maintenance of the structure of an African savanna soil has been investigated in a 28-month field experiment. Changes of aggregate size distribution, porosity and aggregate stability were analysed in undisturbed soil monoliths from which earthworms had been removed and which had then been submitted to four treatments: (1) recolonization by natural savanna fauna (control, 12 replicates), (2) introduction of a population of Millsonia anomala, a large earthworm (13 replicates), (3) introduction of a population of small filiform eudrilid earthworms (13 replicates), and (4) no recolonization by earthworms (12 replicates). At the end of the experiment, significant differences were observed among the four treatments. M. anomala was responsible for the formation of macroaggregates > 5.0 mm whereas eudrilid earthworms egested smaller aggregates (0.5–5 mm). The results suggest that the presence of both types of worms is necessary to maintain the natural soil structure. If one or both types of worm are excluded from the soil, the initial structure is greatly affected.