THE ROLE OF EARTHWORMS IN THE FORMATION OF SANDY SURFACE SOILS IN A TROPICAL FOREST IN IVORY-COAST

Soils with relatively sandy topsoils over clayey subsoils are widespre ad, but their genesis is not always clear. We tested the hypothesis th at earthworms stimulate the formation of the sandy surface soils by pr oducing clay-rich worm casts that are susceptible to erosion, in an un disturbed forested watershed in southwestern Ivory Coast. Soils of the watershed were strongly weathered and acidic. Their contents of irons tone gravel and of clay, organic matter and plant nutrients decrease i n a downslope direction. Production of worm casts increased in the sam e direction from 0.1 to 0.2 kg m(-2) yr(-1) near the top of the waters hed to between 2.3 and 9.6 kg m(-2) yr(-1) near the valley bottom, wit h an areal average of 3.2 kg m(-2) yr(-1). Casts were invariably riche r in clay and silt, organic matter and N, P, Ca and Mg than nearby bul k surface soil. Casts were easily disintegrated by rain splash and ove rland flow and cast material contributed to the 0.12 kg m(-2) yr(-1) o f highly organic suspended sediment removed by surface water drainage. Selective erosion of worm casts was probably more important than eluv iation of clay in removing clay from the surface soils. Increasing pro duction and disintegration of casts with decreasing elevation probably played a major role in lowering contents of clay and nutrients in the surface soils in the lower parts of the area.