The restoration by earthworms (Megascolecidae) of the structure of a 2.0 mm-sieved tropical ferruginous soil (Ferralsol, FAO) has been studied at Lamto (Cote d'Ivoire). Soil monoliths (30 x 30 x 25 cm) have been taken in a shrub savanna, 2.0 mm-sieved, homogenized and put back m the field. Then, they have been subjected to three treatments: no fauna (control), original savanna fauna and Millsonia anomala Omodeo (Megascolecidae) alone (5 g/bloc). The distribution of aggregate size classes has been analysed in time (for 30 months) and depth (5 layers from 0 to 25 cm). A micromorphological study using thin sections has been realized, associated with bulk density measurements and 2-5 mm class aggregates have been submitted to stability tests. The results show an important effect of earthworms in the restoration of original macroaggregated structure by increasing aggregation. In the absence of earthworms, the formation of macroaggregates > 2.0 mm is very limited and may be due to dry-wet cycles and microbial activity. After 30 months of experiment, aggregates > 2.0 mm only represent 12.9% of soil. Because of a low density due to dry climatic conditions before and during the study, original savanna fauna had not a very important effect during the first year of experiment. Aggregation increased slowly: 12.0% of soil as aggregates > 2.0 mm after 14 months of experiment. Then this amount of large aggregates increased more rapidly (49.9% after 30 months). The most important aggregation is realized in treatment with M. anomala: 31.7% after 6 months of experiment and 60.6% after 30 months. The observation of thin sections emphasizes and specifies the role of M. anomala in this aggregation. Aggregates created in presence of earthworms are more water-stable than those created in absence of earthworms.
