CO2 fluxes and carbon sequestration within eucalypt stands in Congo

Since October 2000, CO2 and water fluxes are measured within a young eucalypt stand in Congo. In this experiment, our main objective is to derive the net carbon ecosystem exchange (carbon sequestration) from continuous eddy flux measurements, and to compare these estimates to those obtained from 1) measurements of Net Primary Productivity (NPP) and soil heterotrophic respiration, and 2) measurements of carbon stocks (soil and biomass) and their variations over a chronosequence. Carbon stocks and soil respiration measurements were obtained over a chronosequence that includes 6 stands from 6 months up to 10 years within a 43000 ha massif of clonal eucalypt plantations established around Pointe Noire (4°S 12°E, Republic of Congo) over poor, sandy, and highly desaturated soils previously covered by littoral savannas. The Eddy correlation measurements were obtained from the top of a tower erected within the 3 year-old stand of the chronosequence. Soil CO2 efflux (monitored with a respiration chamber connected to a portable Li6200 infrared gas analyzer) exhibited strong seasonal variations, reflecting the seasonal changes in soil water content. Maximum values were obtained during the wet season, while minimum values were obtained in September-October at the end of the dry season (that lasts from June to October). Annual soil CO2 efflux was 11.8 t C/ha at Eddy correlation site, and 16.7 t C/ha at the 10 year old stand. At each site, good relationships were obtained between soil volumetric water content (measured from 0 to 6 cm from the surface) and soil respiration. The soil moisture effect on soil respiration was easily described by a 3 parameters equation. Rhizospheric and heterotrophic contributions to total soil CO2 efflux were estimated from comparison of soil CO2 efflux measured over trenched plots and soil CO2 fluxes measured over non-trenched plots: at the eddy correlation site (3 year-old stand), root respiration contributed to 26% of total annual CO2 efflux. First results obtained at other stands indicate that root contribution to total soil C02 efflux increases with stand age (e.g. around 57% at the 6 year-old stand), probably as a result of root biomass increase with stand age. Similar to soil CO2 efflux, NEE measured by eddy correlation exhibited strong seasonal variations with lowest NEE values (highest CO2 uptake) obtained during the wet season, when LAI and REW (relative extractable water) were maximum. At this time, minimum diurnal peaks of NEE were typically about -25 micromol m-2 s-1. By contrast, minimum peaks obtained at the end of the dry season were about - 12 micromol m-2 s-1. Monthly NEP ranged from -85.3 g C m-2 up to 29.8 g C m-2. This positive value (net carbon emission) has been observed at the transition between the dry and the wet season, and resulted from a faster increase of ecosystem respiration (estimated from nocturnal NEE measurements) than photosynthesis, after the first rains. Daily gross primary production (GPP) was found to be strongly correlated with daily actual evapo-transpiration (AET), resulting in water use efficiencies (WUE-GPP) of about 5.2 g DM/kg H2O. For the two-year period covered by eddy correlation measurements (October 2000 to October 2002), mean annual net C exchange (NEP) was -370 g C m-2/year. This corresponds to a net carbon uptake by the stands, but represents a small fraction of GPP (-1990 g C m-2/year), due to important carbon loss by ecosystem respiration (1620 g C m-2/year). Aboveground respiration, root respiration, and heterotrophic soil respiration represented 27% (438 g C m-2/year), 19% (307 g C m-2/year) and 54% (874 g C m-2/year) of ecosystem respiration, respectively. Over the same period NPP was -1203 g C m-2/year (-676 g C m-2/year for total tree biomass increment, and -527 g C m-2/year for fine root turnover and litter production). Summing NPP and heterotrophic respiration provides another estimates of NEP (-330 g C m-2/year), slightly lower to the one obtain