Résumé
Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO 2 efflux (F s ), a major flux in the global C cycle. We tested the hypothesis that F s increases with stand age after afforestation, due to an increasing input of fresh organic matter to the forest floor. In a Eucalyptus plantation established on coastal savannahs in Congo, bimonthly measurements of F s were carried out for 1 year on three adjacent stands aged 0.9, 4.4 and 13.7 years and presenting similar growth patterns. Litterfall and litter accumulation on the forest floor were quantified over a chronosequence. Equations were derived to estimate the contribution of litter decomposition to F s throughout the rotation. Litterfall increased with stand age after savannah afforestation. F s , that was strongly correlated on a seasonal basis with soil water content (SWC) in all stands, decreased between ages 0.9 year and 4.4 years due to savannah residue depletion, and increased between ages 4.4 years and 13.7 years, mainly because of an increasing amount of decomposing eucalypt litter. The aboveground litter layer therefore appeared as a major source of CO 2 , whose contribution to F s in old stands was estimated to be about four times higher than that of the eucalypt-derived soil organic C pool. The high litter contribution to F s in older stands might explain why 13.7 years-old stand F s was limited by moisture all year round whereas SWC did not limit F s for large parts of the year in the youngest stands.