Energy and Climate Change Mitigation Benefits of Faidherbia albida Agroforestry in Ethiopia.

Faidherbia albida trees have an unusual phenology, leafing out during the dry season and shedding leaves at the start of the rainy season. When integrated into agroforestry cropping systems, they provide substantial sustainability benefits, including adding plant nutrients and organic matter to the soil through leaf drop. Dormant during the cropping season, Faidherbia albida does not compete with the crop for light, water or nutrients and contributes nitrogen to the soil under its canopy, enhancing crop productivity. This nitrogen benefit is analyzed in relation to its equivalent in industrially manufactured urea nitrogen fertilizer, expressed in terms of the energy required to manufacture and transport an equivalent quantity of urea fertilizer. This is, in effect, a substitution of solar energy that the Faidherbia albida trees use to obtain nitrogen from the atmosphere, for the fossil fuels used in the industrial manufacture and transport of the urea fertilizer. This energy contribution by the tree, within the food energy and water system, enhances the food production potential of the system, as yields are often increased, and the resilience of the system is enhanced because the soil organic matter increases available water for the plants. This energy contribution to the Ethiopian farming system is estimated as 3.48 GJ ha -1 year -1 , based on the nitrogen contribution under full canopy cover. Additions of energy to agricultural systems frequently increase productivity and such increases can be realized through the addition of energy provided by Faidherbia agroforestry. Greenhouse gas emissions are avoided by the substitution of solar energy for fossil fuel energy, a climate change mitigation benefit estimated as 0.116 tons CO 2 ha -1 year -1 . This contribution to climate change mitigation is fundamentally different from sequestration of carbon in biomass or soil organic matter. It is a permanently avoided emission of carbon dioxide into the atmosphere, associated with a particular cropping year, and is not reversible, unlike the benefit of carbon stored in biomass or soil organic matter, that could be released back into the atmosphere. The potential extent of Faidherbia albida agroforestry is substantial and the potential climate change mitigation benefits of its widespread use are great.