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Age-related changes in litter inputs explain annual trends in soil CO2 effluxes over a full Eucalyptus rotation after afforestation of a tropical savannah
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Edité par CCSD ; Springer Verlag -
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.
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