Strong seasonal variability in African carbon source/sink relationship is considered the continent’s most significant contribution to the global carbon cycle. Alternating dry and wet periods dictate ecosystem carbon exchange and productivity of tropical African savannah. We examined the seasonal and daily trends in ecosystem CO2 exchange in the herbaceous layer of a humid Kenyan savannah devoid of grazing. Microclimate, soil moisture, soil and tissue nitrogen, aboveground biomass and carbon dioxide fluxes were measured. The ecosystem carbon dioxide fluxes followed the seasonal rainfall pattern and were strongly correlated to soil water content. Peak mean carbon dioxide fluxes were 3.21±0.99, 8.21±1.02 and 4.67±1.06 µmolm-2 s -1 during drought and -13.86±1.48, 15.03±0.98, and 27.73±1.47 µmolm-2 s -1 during wet periods for net ecosystem carbon dioxide exchange, ecosystem respiration and gross primary production respectively. At daily scale, net ecosystem carbon dioxide exchange increased with increasing photosynthetic photon flux density under wet but decreased with increasing vapour pressure deficit under dry conditions. Ecosystem respiration increased with increasing soil temperature during wet but decreased with increasing soil temperature during drought. Our results point to the overriding role of soil moisture in the ecosystem carbon dioxide exchange processes of this savannah