Effects of a tropical geophageous earthworm, M. anomala (Megascolecidae), on soil characteristics and production of a yam crop in Ivory Coast

This study was an attempt to validate under field conditions some results of the effects of earthworms on soil organic matter dynamics, obtained under confined circumstances, and on enhanced plant production. Yams were grown in the presence or absence of the tropical endogeic earthworm Millsonia anomala (Omodeo and Vaillaud, 1967) for 3 y. Field experiments were conducted in the middle of Ivory Coast using experimental plots isolated by PVC sheets. At the beginning of the cropping sequence, earthworm biomass was adjusted to 25 g m−2, but as the study progressed the capacity of the soil to sustain populations decreased. By the third year, biomass had decreased to 3 g m−2. The presence of worms affected soil structure by increasing the proportion of large aggregates that were mainly ageing casts. In the presence of M. anomala, soil C mineralisation decreased by 5% after 3 y, but distribution of C among granulometric fractions of soil organic matter was the same in the two treatments. Yam tuber production was increased by 20%, 0% and 53% at the first, second and third crop, respectively, in presence of M. anomala. The earthworms affected both soil and plants but the origin of these effects were unclear.