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In tropical savannas where soils are generally sandy and nutrient poor, organic farming associated with enhanced soil biological activity may result in increased nutrient availability. Therefore, legumes have been introduced in the humid savanna zone of Côte d'Ivoire, owing to their ability to fix atmospheric N and to continually supply soil with great quantity of organic materials in relatively short time. The main objective of this study was to assess the influence of two legume (Cajanus cajan and Lablab purpureus) cultivations on earthworm communities and P and N availability. Trials were carried out under farmers' field conditions; C. cajan was planted on savanna soils (trial 1) while L. purpureus was established on new Chromolaena odorata-dominated fallow soils (trial 2). Native vegetations were considered as controls. Changes in soil properties (earthworm abundance and diversity, microbial biomass carbon (MBC), and plant available P and N) were assessed using the biosequential sampling. After 1 year, both the legume stands showed a significantly higher density of earthworms, compared with the respective controls. This trend was linked to an increase in the abundance of the detritivores Dichogaster baeri Sciacchitano 1952 and Dichogaster saliens Beddard 1893, and the polyhumic Stuhlmannia zielae Omodeo 1963. Equally, legume had beneficial impacts on the average number of earthworm species, the Shannon-Weaver index of diversity and MBC in savanna (trial 1). Available P and ammonium significantly increased under both legume cultivations and were significantly and concurrently linked to litter quality and earthworm activities as shown by multiple regressions. As a result, legumes could improve nutrient availability in the sandy soils of central Côte d'Ivoire by positively affecting soil biological activity and this could bring farmers to cultivate crops on savanna lands.

The conversion of natural forest to oil palm (Elaeis guineensis Jacq.) plantation is perceived as a threat to biodiversity conservation. The aims of this investigation were to assess variation in soil physico-chemical parameters and macroinvertebrate structures following the conversion of secondary forests to oil palm plantations, and to understand what to expect with the aging of the plantations. We hypothesised that soil properties would be improved with respect to biodiversity conservation with the aging of the oil palm plantations. Sampling was carried out in secondary forests, and in 13-, 20- and 39-year-old oil palm plantations at the La Mé Station, Côte d’Ivoire. Three sampling areas were established in each land-use type and age class, for a total of 12 sampling areas. Over a 50-m transect, litter-dwelling macroinvertebrates were sampled by using pitfall traps, and topsoil (0–10 cm) macroinvertebrates by using monoliths (50 cm by 50 cm by 10 cm), following the modified ‘Tropical soil biology and fertility’ method. Soil physical and chemical parameters were characterised. The results showed that the conversion of secondary forests to oil palm plantations was characterised by changes in soil macroinvertebrate density (–30%), taxonomic richness (–37%) and functional groups (–17%), as well as soil bulk density (+14%) and water content (+16%), after ~39 years of conversion. Soil organic carbon, total nitrogen and organic matter increased with the aging of plantations, and reached conditions similar to those of secondary forests. The human interference imposed on macroinvertebrate communities during site preparation and planting was not compensated by the reduction in the soil-degradation index over time. The results have implications for implementation of conservation agriculture and the related management practices.

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