In tropical areas, where crop production is limited by low soil quality, the development of techniques improving soil fertility without damage to the environment is a priority. In French Guiana, we used subsistance farmer plots on poor acidic soils to test the effect of different organic amendments, bitter manioc peel (M), sawdust (Sw) and charcoal (Ch), on soil nutrient content, earthworm abundance and yard-long bean (Vigna unguiculata sesquipedalis) production. The peregrine Pontoscolex corethrurus was the only earthworm species found. Pod production and plant growth were lowest in unamended soil. The application of a mixture of manioc peel and charcoal (M+Ch) improved legume production as compared with other organic mixtures. It combined favourable effects of manioc peel and charcoal. Manioc peel improved soil fertility through its low C:N ratio and its high P content, while charcoal decreased soil acidity and exchangeable Al and increased Ca and Mg availability, thus alleviating possible toxic effects of Al on plant growth. The M+Ch treatment was favourable to P. corethrurus, the juvenile population of which reaching a size comparable to that of the nearby uncultivated soil. The application of a mixture of manioc peel and charcoal, by improving crop production and soil fertility and enhancing earthworm activity, could be a potentially efficient organic manure for legume production in tropical areas where manioc is cultivated under slash-and-burn shifting agriculture.

The endogeic earthworm Pontoscolex corethrurus (Glossoscolecidae) is a peregrine species commonly found in tropical lands cleared by man for cultivation. We compared the charcoal consumption and casting activity of a population of P. corethrurus from a cultivated area under repeated slash-and-burn (fallow population) with that of a population living in a field cultivated after recent burning of a mature forest (forest population). Their cast production was measured in containers in the presence of pure charcoal, soil of fallow and forest origin, or a mixture of charcoal and soil. The forest population defecated less in pure charcoal than in forest soil, whereas the reverse was observed in the fallow population. When living in fallow soil, both populations defecated more at the surface of a mixture of charcoal and soil than at the surface of pure soil (x 2 and x 3 with fallow and forest population, respectively). In forest soil, both populations showed an increased charcoal consumption (x 12). In the light of these experiments, we hypothesized that an adaptation of P. corethrurus to charcoal and fallow soil exists, supporting the observed distribution of this earthworm species in tropical open lands.

The geophagous earthworm Pontoscolex corethrurus is frequently found in burnt tropical soils where charcoal plays an important role in soil fertility. We studied the burrowing activity of this species in two-dimensional microcosms with one half filled with soil and the other with a 3:2 (w:w) mixture of charcoal and soil (CHAR+soil). We measured the volume of empty burrows and those filled with black or brown casts in both substrates, as well as the initial and final fresh weights of the worms. The correlation between brown cast production and both initial and final fresh weights of the worms, reinforced by the presence of feeding cavities in soil but not in CHAR+soil, suggests that P. corethrurus would ingest soil to fulfill its nutrient requirements, in contrast to charcoal which was ingested for other purposes. We observed that at equal burrow volume created in the two substrates, P. corethrurus produced smaller black casts than brown casts, suggesting that burrows were created in CHAR+soil mainly by pushing aside the particles of this lighter substrate.