Abstract. This study gives an outlook on the carbon balance of Sub-Saharan Africa (SSA) by presenting a summary of currently available results from the project CarboAfrica (namely net ecosystem productivity and emissions from fires, deforestation and forest degradation, by field and model estimates) supplemented by bibliographic data and compared with a new synthesis of the data from national communications to UNFCCC. According to these preliminary estimates the biogenic carbon balance of SSA varies from 0.16 Pg C y −1 to a much higher sink of 1.00 Pg C y −1 (depending on the source data). Models estimates would give an unrealistic sink of 3.23 Pg C y −1 , confirming their current inadequacy when applied to Africa. The carbon uptake by forests and savannas (0.34 and 1.89 Pg C y −1 , respectively,) are the main contributors to the resulting sink. Fires (0.72 Pg C y −1 ) and deforestation (0.25 Pg C y −1 ) are the main contributors to the SSA carbon emissions, while the agricultural sector and forest degradation contributes only with 0.12 and 0.08 Pg C y −1 , respectively. Savannas play a major role in shaping the SSA carbon balance, due to their large extension, their fire regime, and their strong interannual NEP variability, but they are also a major uncertainty in the overall budget. Even if fossil fuelCorrespondence to: A. Bombelli (bombelli@unitus.it) emissions from SSA are relative low, they can be crucial in defining the sign of the overall SSA carbon balance by reducing the natural sink potential, especially in the future. This paper shows that Africa plays a key role in the global carbon cycle system and probably could have a potential for carbon sequestration higher than expected, even if still highly uncertain. Further investigations are needed, particularly to better address the role of savannas and tropical forests and to improve biogeochemical models. The CarboAfrica network of carbon measurements could provide future unique data sets for better estimating the African carbon balance.

Abstract. The impact of fire on soil fluxes of CO 2 , CH 4 and N 2 O was investigated in a tropical grassland in Congo Brazzaville during two field campaigns in [2007][2008]. The first campaign was conducted in the middle of the dry season and the second at the end of the growing season, respectively one and eight months after burning. Gas fluxes and several soil parameters were measured in each campaign from burned plots and from a close-by control area preserved from fire. Rain events were simulated at each campaign to evaluate the magnitude and duration of the generated gas flux pulses. In laboratory experiments, soil samples from field plots were analysed for microbial biomass, net N mineralization, net nitrification, N 2 O, NO and CO 2 emissions under different water and temperature soil regimes. One month after burning, field CO 2 emissions were significantly lower in burned plots than in the control plots, the average daily CH 4 flux shifted from net emission in the unburned area to net consumption in burned plots, no significant effect of fire was observed on soil N 2 O fluxes. Eight months after burning, the average daily fluxes of CO 2 , CH 4 and N 2 O measured in control and burned plots were not significantly different. In laboratory, N 2 O fluxes from soil of burned plots were significantly higher than fluxes from soil of unburned plots only above 70% of maximum soil water holding capacity; this was never attained in the field even after rain simulation. HigherCorrespondence to: S. Castaldi (simona.castaldi@unina2.it) NO emissions were measured in the lab in soil from burned plots at both 10% and 50% of maximum soil water holding capacity. Increasing the incubation temperature from 25 • C to 37 • C negatively affected microbial growth, mineralization and nitrification activities but enhanced N 2 O and CO 2 production. Results indicate that fire did not increase postburning soil GHG emissions in this tropical grasslands characterized by acidic, well drained and nutrient-poor soil.

Abstract. This study presents a summary overview of the carbon balance of Sub-Saharan Africa (SSA) by synthesizing the available data from national communications to UNFCCC and first results from the project CarboAfrica (net ecosystem productivity and emissions from fires, deforestation and forest degradation, by field and model estimates). According to these preliminary estimates the overall carbon balance of SSA varies from 0.43 Pg C y−1 (using in situ measurements for savanna NEP) to a much higher sink of 2.53 Pg C y−1 (using model estimates for savanna NEP). UNFCCC estimates lead to a moderate carbon sink of 0.58 Pg C y−1. Excluding anthropogenic disturbance and intrinsic episodic events, the carbon uptake by forests (0.98 Pg C y−1) and savannas (from 1.38 to 3.48 Pg C y−1, depending on the used methodology) are the main components of the SSA sink effect. Fires (0.72 Pg C y−1), deforestation (0.25 Pg C y−1) and forest degradation (0.77 Pg C y−1) are the main contributors to the SSA carbon emissions, while the agricultural sector contributes only with 0.12 Pg C y−1. Notably, the impact of forest degradation is higher than that caused by deforestation, and the SSA forest net carbon balance is close to equilibrium. Savannas play a major role in shaping the SSA carbon balance, due to their large areal extent, their fire regime, and their strong interannual NEP variability, but they are also a major uncertainty in the overall budget. This paper shows that Africa plays a key role in the global carbon cycle system and probably could have a potential for carbon sequestration higher than expected, even if still highly uncertain. Further investigations are needed, particularly to better address the role of savannas and tropical forests. The current CarboAfrica network of carbon measurements could provide future unique data sets for better estimating the African carbon balance.

Grassland savannah ecosystems subject to frequent fires are considered to have an almost neutral carbon balance, as the C released during burning mostly balance the C fixed by the photosynthetic process. However, burning might modify the net soil-atmosphere exchange of GHGs in the post burning phase so that the radiative balance of the site might shift from neutrality. In the present study the impact of fire on soil fluxes of CO₂, CH₄ and N₂O was investigated in a grassland savannah (Congo Brazzaville) where high frequency burning is the typical management form of the region. An area was preserved for one season from annual burning and was used as "unburned" treatment. Two field campaigns were carried on at different time length from the fire event, 1 month, in the middle of the dry season, and 8 months after, at the end of the growing season. CO₂, CH₄ and N₂O fluxes, as well as several soil parameters, were measured in each campaign from burned and unburned plots. Rain events were simulated at each campaign to evaluate magnitude and length of the generated GHG flux pulses. In laboratory experiments, on soil samples from the two treatments, microbial biomass, net N mineralization, net nitrification, N₂O, NO and CO₂ emissions were analyzed in function of soil water and/or temperature variations. Results showed that fire had a significant effect on GHG fluxes but the effect was transient, as after 8 months differences between treatments were no longer significant. One month after burning CO₂ soil emissions were significantly lower in the burned plots, CH₄ fluxes were dominated by net emissions rather than net consumption in the unburned area and fire shifted the CH₄ flux distribution towards more negative values. No significant effect of fire was observed in the field on N₂O fluxes. It was assumed that the low water content was the main limiting factor as in fact laboratory data showed that only above 75% of water saturation, N₂O emissions increased sharply and more strongly in the soil from burned plots. This soil water content was hardly reached in the field even in the watered plots. Burned also stimulated NO production in the laboratory, which was more evident at low water content. Differently from N₂O, 25% of water saturation was sufficient to significantly stimulate CO₂ production in the laboratory and rain simulation in the field stimulated soil respiration. However in the laboratory the highest fluxes were measured in burned soil whereas in the field the opposite was observed. Increasing the incubation temperature from 25 °C to 37 °C affected negatively microbial growth and activities (mineralization and nitrification) but stimulated gas production (N₂O and CO₂). Overall, data indicate that fire would have a reductive or null impact on soil GHG emissions in savannah sites presenting similar soil characteristics (acidic, wel...

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