Carbon dioxide fluxes from contrasting ecosystems in the Sudanian Savanna in West Africa

Abstract

BackgroundThe terrestrial land surface in West Africa is made up of several types of savanna ecosystems differing in land use changes which modulate gas exchanges between their vegetation and the overlying atmosphere. This study compares diurnal and seasonal estimates of CO2 fluxes from three contrasting ecosystems, a grassland, a mixture of fallow and cropland, and nature reserve in the Sudanian Savanna and relate them to water availability and land use characteristics.ResultsOver the study period, and for the three study sites, low soil moisture availability, high vapour pressure deficit and low ecosystem respiration were prevalent during the dry season (November to March), but the contrary occurred during the rainy season (May to October). Carbon uptake predominantly took place in the rainy season, while net carbon efflux occurred in the dry season as well as the dry to wet and wet to dry transition periods (AM and ND) respectively. Carbon uptake decreased in the order of the nature reserve, a mixture of fallow and cropland, and grassland. Only the nature reserve ecosystem at the Nazinga Park served as a net sink of CO2, mostly by virtue of a several times larger carbon uptake and ecosystem water use efficiency during the rainy season than at the other sites. These differences were influenced by albedo, LAI, EWUE, PPFD and climatology during the period of study.ConclusionThese results suggest that land use characteristics affect plant physiological processes that lead to flux exchanges over the Sudanian Savanna ecosystems. It affects the diurnal, seasonal and annual changes in NEE and its composite signals, GPP and RE. GPP and NEE were generally related as NEE scaled with photosynthesis with higher CO2 assimilation leading to higher GPP. However, CO2 effluxes over the study period suggest that besides biomass regrowth, other processes, most likely from the soil might have also contributed to the enhancement of ecosystem respiration.

Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa)

“…This was probably due to the differences between the soil evaporation rates, soil characteristics, deeps infiltration, and importance of the runoffs in relation with the vegetation types at the two sites. These results were in accordance with other findings reported between some sites more and less near each other in West Africa by other authors, especially concerning the rainfall regimes (Ago et al 2005;Boulain et al 2009;Seghieri et al 2009;Merbold et al 2009;Quansah et al 2015). It appears reasonable to consider both meteorological conditions at the two sites as similar overall.…”

“…This suggests that the ER was mainly controlled by the soil water availability at the two sites as reported for similar ecosystems in Africa by other authors (Williams and Albertson 2004;Williams et al 2009;Ago et al 2014). In contrast, other studies reported for several dry West African sites (annual sum rainfall < 1000 mm) that the ER was primarily driven by the temperature and additionally by the soil water content (Brümmer et al 2008;Merbold et al 2009;Quansah et al 2015). The uncertainties are the 95 % confidence intervals T A air temperature, T S soil temperature, VPD vapor pressure deficit, SWC soil water content at 10 cm depth, LAI leaf area index, F forest, S savannah…”

“…Overall, period I and II were characterized by higher radiation (Q), higher air temperature (T A ), higher vapor pressure deficit (VPD), and lower soil water content (SWC) at 10 cm depth. Compared to period II, the relative lower radiation during period I can be mostly explained by an available radiation in the atmosphere top and peaks are associated with Harmattan events which transport the aerosols to the West Africa region (Nicholson 2013;Mamadou et al 2014;Quansah et al 2015). During the drought periods (I and II), the isolated rainy events occurred (first one of April), and~20 % of the rainy days were observed with more or less 17-18 % of the 2009 annual rainfall (Tables 1 and 4).…”

“…We used the parameterizations to describe the fluxes responses to the main meteorological and edaphic driving factors as applied by several authors in Africa (Merbold et al 2009;Ago et al 2014Ago et al , 2015Tagesson et al 2015;Quansah et al 2015). They were different for the daytime and nighttime fluxes.…”

“…Thus, some investigations on responses of CO 2 fluxes and ecosystems properties to main meteorological, physiological, ecological, and anthropogenic factors are necessary. In addition, only a few studies have been reported on long-term CO 2 fluxes and carbon sequestrations in West Africa by several frameworks such as CarboAfrica project and AMMA-CATCH "African Monsoon Multidisciplinary Analysis" and "Couplage de l'Atmosphère Tropicale et du Cycle Hydrologique" observatory, which contributes significantly to the FluxNet network database (http://fluxnet.ornl.gov) (Brümmer et al 2008;Ago et al 2014Ago et al , 2015Tagesson et al 2015;Quansah et al 2015). Our study analyzed the CO 2 fluxes that were measured over a cultivated savannah and a protected forest under a Sudanian climate in northern Benin.…”