Abstract
Land use changes such as savannah afforestation with eucalypts impact the soil carbon (C) balance, therefore affecting soil CO2 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 CO2, 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.
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