Ten years of carbon dioxide fluxes and carbon balance at a mixed cultivated savannah and an open forest in a tropical humid climate in West Africa

The huge pressure on tropical forests due to agricultural expansion threats the capacity of the West African region to sequester atmospheric CO₂, a region which is supposed to account for 20% of CO₂ emissions of the whole continental tropical belt. Yet, the scarcity of eddy covariance measurements in the tropical humid African region has led to significant challenges in understanding the carbon sequestration potential of forest ecosystems and more broadly the amount of CO₂ which will be lost when they are converted into culture. Taking advantage of two nearby eddy covariance sites established in Northern Benin, a mixed crop savannah (Nalohou, lat. 9.74^°N, long. 1.60^°E) and a clear forest (Bellefoungou, lat. 9.79°N, long.1.72°E), this study compares their net ecosystem exchange (NEE) dynamics and their carbon balance, using data spanning from 2007 to 2017. Driven by the precipitation pattern, the CO₂ dynamics display strong seasonality above both ecosystems, with moderate uptakes during extreme precipitation years. We determined the optimal respiration model for both ecosystems, enabling the partitioning of NEE fluxes into total ecosystem respiration (Reco) and gross primary production (GPP).  Soil moisture was found to be the main driver of nighttime CO₂ emissions, with a sigmoidal model the most appropriate for representing Reco. When using soil moisture as an input in the ecosystem respiration model for partitioning NEE, we found, based on the ten years dataset, an average annual NEE of  -512 ± 69 g C m⁻² y⁻¹ at the forest site and of -202 ± 53 g C m⁻² y⁻¹ at the mixed crop site. Finally, these tropical humid ecosystems were observed to be, during all years analyzed, a net sink of atmospheric CO₂, showing that forest CO₂ sequestration is 2.5 times the cultivated site one.  These results constitute a paramount information for earth system models regarding carbon budget of these typical and understudied African ecosystems.