Age-related changes in litter inputs explain annual trends in soil CO2 effluxes over a full Eucalyptus rotation after afforestation of a tropical savannah

Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO2 efflux (Fs), a major flux in the global C cycle. We tested the hypothesis that Fs 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 Fs 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 Fs throughout the rotation. Litterfall increased with stand age after savannah afforestation. Fs, 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 CO2, whose contribution to Fs 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 Fs in older stands might explain why 13. 7 years-old stand Fs was limited by moisture all year round whereas SWC did not limit Fs for large parts of the year in the youngest stands. © 2011 Springer Science+Business Media B.V.