THE PHYSICAL STRUCTURE OF CASTS OF MILLSONIA-ANOMALA (OLIGOCHAETA, MEGASCOLECIDAE) IN SHRUB SAVANNA SOILS (COTE-DIVOIRE)

A tropical geophagous earthworm species, Millsonia anomala, is mainly responsible for the formation and the conservation of the structure of the upper horizons of soil (Ferralsol) at Lamto (Cote d'Ivoire). Its action on soil organic matter dynamics is important as organic matter in their water-stable casts is protected against mineralization. This study aims at describing the physical structure of these casts in order to explain the protection of organic matter through the cast structure and to emphasize the role of earthworms on the soil structure. The pore size distribution (mercury porosimetry) and the bulk density of casts were measured and casts were studied using thin sections and scanning electron microscopy observations. Casts collected at the surface of a shrub savanna soil and from earthworm cultures were analysed. The results show that casts of M. anomala are very dense (bulk density ranges from 1.8 to 2.0 g cm-3) and that they have an important volume of pores with a diameter of ca. 10 mum. On the outside. these casts display a dense layer of ca. 25 mum thick. This cortex consists of fine particles and can be expected to impede water and air movements. This particular physical structure may explain the protection of organic matter in casts, that has been observed by other authors.