REGULATION OF SOIL-STRUCTURE BY GEOPHAGOUS EARTHWORM ACTIVITIES IN HUMID SAVANNAS OF COTE-DIVOIRE

The role of endogeic earthworms in the maintenance of the structure of an African savanna soil has been investigated in a 28-month field exp eriment. Changes of aggregate size distribution, porosity and aggregat e stability were analysed in undisturbed soil monoliths from which ear thworms had been removed and which had then been submitted to four tre atments: (1) recolonization by natural savanna fauna (control, 12 repl icates), (2) introduction of a population of Millsonia anomala, a larg e earthworm (13 replicates), (3) introduction of a population of small filiform eudrilid earth Norms (13 replicates), and (4) no recolonizat ion by earthworms (12 replicates). At the end of the experiment, signi ficant 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 result s suggest that the presence of both types of worms is necessary to mai ntain the natural soil structure. If one or both types of worm are exc luded from the soil, the initial structure is greatly affected. (C) 19 97 Elsevier Science Ltd.