Computing the Net Primary Productivity for a Savanna- Dominated Ecosystem Using Stable Isotopes: A Case Study of the Volta River Basin

The hydrologic systems and the terrestrial ecosystem of the Volta river basin in West Africa, play important role in the carbon cycle. This is so because of the coupling of water vapour release and CO2 uptake during photosynthesis, expressed as water use efficiency or transpiration ratio. Hydrologic and land-cover data, together with stable isotope ratio measurements of d18O and dD, and data from the global network of isotopes in precipitation (GNIP) are used to determine the net primary productivity (NPP) of the savanna-dominated ecosystem. The d18O and dD values in the Volta rivers range from -4.72 to 2.37 mm-l and from -35.28 to 9.30 mm-1 SMOW, respectively. The results indicate that the vegetation is supported by 380 km3 of rainfall, out of which 50% is returned to the atmosphere via plant transpiration. Associated with annual transpiration is the NPP of 0.170 × 1015gCyr–1 or 428 gCm-2 from the terrestrial ecosystem. Modelled estimates of heterotrophic soil respiration in this study slightly exceeded the NPP estimates, implying a small source of CO2 to the atmosphere. This condition does not favour the postulated existence of a major sink of atmospheric CO2 in the Volta basin.