Résumé
Afforestation of savannah with cocoa agroforestry systems (cAFS) is a common farmer practice in Cameroon previously described as a sustainable production option. Nevertheless, the effects of afforestation of savannah with cAFS on C turnover and content, and the factors controlling C accumulation and stabilization are unknown. Different systems settled on savannah were compared: cropland (≈ 5 years old), cocoa monoculture (≈10 years old) and cAFS (from 20 to 60 years old) shaded by different tree species (Albizia adianthifolia, Canarium schweinfurthii, Dacryodes edulis, Milicia excelsa, Ceiba pentandra). We used savannah and nearby forest patches as controls. Soils were orthic ferralsols with 9-15% of clay content. Soil analysis was performed on the 0-10 cm soil layer for: organic C content, C distribution in soil particle size fractions (0-20 µm, 20-50 µm, and 50-2000 µm), and nutrient contents. Soil delta13C was analysed for studying how the change from savannah grasses (C4 plants) to other vegetation (C3 plants) affected soil C turnover. The amount of annual litter input and its nutrient content were also analysed. Conversion of savannah to cAFS significantly increased soil C to the same level as soil C under nearby forests (Figure 1). Conversion of savannah to annual cropland or cocoa monoculture resulted in a non-significant decrease in soil C. After conversion of savannah to other land uses, more than 70% of soil C derived from C4 plants was lost within 10 years and then remained almost unchanged. Contrastingly, soil C derived from C3 plants increased significantly in cAFS. The C accumulation occurred both in the 50-2000 µm and 0-20 µm soil fractions, and was linked to cumulative higher litter inputs in cAFS than in other systems. Soil C under the different shade tree species was positively linked to soil pH, exch. Ca2+ and litter Ca content. Afforestation of savannah with cAFS appears as a valuable option for soil carbon sequestration.