EFFECTS OF A TROPICAL GEOPHAGOUS EARTHWORM, MILLSONIA-ANOMALA, ON SOME SOIL CHARACTERISTICS, ON MAIZE-RESIDUE DECOMPOSITION AND ON MAIZE PRODUCTION IN IVORY-COAST

Maize was grown under field conditions in the presence and absence of the tropical endogeic earthworm Millsania anomala (Omodeo and Vaillaud , 1967) in soil of a secondary forest of central Ivory Coast. Experime ntal units were isolated by PVC sheets to limit earthworm movements. D ecomposition and redistribution of nitrogen from maize crop residues i ncorporated in the soil were monitored using N-15-labelled residues. T he density of M. anomala decreased from 50 to 15.9 m(-2) during the cu ltivation period (90 days). Activity of M. anomala significantly modif ied the structure of the soil by increasing the proportion of large ag gregates (over 2 mm in diameter). Maize production was increased by 12 % for stalks and 18% for grains in the presence of earthworms. Nitroge n contained in the maize residue was more efficiently used by plants g rown in the presence of earthworms; the real coefficient of utilisatio n of the organic residue increased from 9 to 11% in the presence of M. anomala. Nonetheless, total nitrogen exportation was not significantl y different between the two treatments. Furthermore, N from the organi c residues left after the cultivation period was less easily assimilat ed by plants in treatments with earthworms.