HR: 1340h
AN: B23C-1504    [Abstracts]
TI: A Comparison Study of CO2 Exchange Within the Sahelian/Sudanian Zone of Africa
AU: * Soegaard, H
EM: hs@geogr.ku.dk
AF: Institute of Geography and Geology, Copenhagen University, Oester voldgade 10, Copenhagen K, 1350, Denmark
AU: Ardoe, J
EM: jonas.ardo@natgeo.lu.se
AF: Physical Geography and Ecosystems Analysis, Lund University, Solvegatan 12, Lund, 22100, Sweden
AB: The paper summarizes some of the major findings from CO2 flux campaigns ranging from Hapex Sahel (Niger 1992) to the presently ongoing Sudan experiment. In total four sites are selected, all with sandy soils, low annual rainfall (250-500 mm), sparse vegetation with a maximum volumetric water holding capacity of approximately 15 %. At all sites the net carbon exchange is measured by eddy covariance technique while the two major components, i.e. the soil respiration and the net carbon assimilation are estimated by combining measurements and modeling. For the soil respiration it is found that the fist rainfall events prior to the proper rainy season generate significance CO2-busts form the wet soil. In general it is found that the soil is strongly controlled by temperature and soil moisture but also available soil carbon should play a significant role. Using the annual maximum leaf area index as a proxy for the available carbon content it is found that the modeling of the soil respiration can be significantly improved . Except for the early seson peaks the maximum respiration level is around 3-4 ƒÝmol m-2 s-1 during the growing season decreasing to 2 ƒÝmol m-2 s-1 during senescence and reaching values close to zero during the dry season. For the CO2 assimilation there are large diurnal fluctuations with downward directed fluxes reaching a maximum level of -20 ƒÝmol m-2 s-1 and upward directed nighttime fluxes typically around 5 ƒÝmol m-2 s-l. The photosynthetic CO2 uptake is modeled by use of a mechanistic model based on published values of the Rubisco capacity. The combined photosynthesis /soil respiration model is used for estimating seasonal carbon budgets and to estimate the environmental control of the CO2 exchange. Besides radiation and LAI, the CO2 assimilation is found to depend on soil moisture and temperature The sensitivity analysis shows that at low soil moisture the assimilation is determined by the water stress function whereas for soil moisture contents above 8 vol. % there is only little control on the assimilation. The temperature dependence confirms that there is an optimal temperature at around 36 oC and the assimilation rate decreases for both increasing and decreasing temperature. It is finally discussed how the extent of the Sahelian/Sudanian zone makes it necessary to combine the flux station data with earth observation technique when aiming at regional estimates.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0430 Computational methods and data processing
DE: 0480 Remote sensing
SC: Biogeosciences [B]
MN: 2007 Fall Meeting