Spatial carbon, nitrogen and phosphorus budget of a village in the West African savana : II. Element flows and functioning of a mixed-farming system

Management of organic resources plays a decisive role in the viability of mixed-farming systems in West African savannas. For this reason in this study carbon (C), nitrogen (N) and phosphorus (P) flows initiated by the different uses of organic resources were quantified in the different land-use systems of a village in southern Senegal. Livestock, crop harvest, and wood and straw collecting were responsible for, respectively, 59, 27 and 14% of the C outflows from the area exploited by the village. Livestock accounted for nearly 80% of C, N and P returns to the soil. As a result of these transfers and of on-site recycling of herbaceous biomass, high C inputs were brought to staple crops in the compound ring. Nitrogen and P depletion of the system amounted to -4 kg N and -1 kg P ha-1 year -1 only when other abiotic flows were included. This study also suggests that population growth is likely to rapidly alter the sustainability of this mixed-farming system at least due to a modification in the balance between the supply of and demand for organic resources.