Abstract
Agriculture must raise productivity while addressing climate change in order to ensure the food security of a growing population. Adding fossil fuel energy to the agricultural system can increase productivity through the use of manufactured fertilizer but creates greenhouse gas emissions. This study quantifies an alternative in which energy is added to the agricultural system through a substitution of solar energy for fossil fuel energy by the tree species Faidherbia albida. This substitution can be quantified as an avoided emission of greenhouse gas, a climate benefit. Faidherbia albida trees have unusual phenology, leafing out during the dry season and shedding leaves in the rainy season. In agroforestry systems, Faidherbia albida adds nutrients and organic matter to the soil through leaf drop, and these are beneficial to the crop growing under the tree canopy. Dormant during the cropping season, they do not compete for light, water or nutrients and contribute nitrogen to the soil under their canopy. This nitrogen benefit is analyzed in relation to an equivalent quantity of urea fertilizer. This is a substitution of solar energy that the trees use to obtain nitrogen from the atmosphere, for the fossil fuels used in the manufacture and transport of urea fertilizer. This energy contribution by the tree, within the food energy and water system, enhances the food production, and resilience of the system, as 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. 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 CO2 ha-1 year-1. This 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 carbon stored in biomass or soil organic matter that could return to the atmosphere. The potential extent of Faidherbia albida agroforestry is substantial and its potential climate change mitigation benefits are great.
Highlights
The relationship between energy inputs and agricultural productivity is a critical aspect of the food-energy-water nexus
The marked increases in crop yield, especially in maize associated with Faidherbia agroforestry (Saka et al, 1994), illustrate a direct link between this tree-based energy input and the potential for enhanced food security, especially since Faidherbia is welladapted to challenging dryland regions such as the Sahel
The adoption of Evergreen Agriculture often associated with F. albida is becoming widespread in Africa, with demonstrated success in improving farmer livelihoods and agricultural sustainability in Ethiopia, Zambia, Malawi, Burkina Faso, and Niger (Garrity et al, 2010) and F. albida is one of the principle species incorporated into this approach
Summary
The relationship between energy inputs and agricultural productivity is a critical aspect of the food-energy-water nexus. Faidherbia albida makes significant additions of nitrogen to the soil annually and these additions can be expressed in terms of an equivalent quantity of urea fertilizer [CTFT (Centre Technique Forestier Tropical), 1989] This nitrogen addition is valuable both as a nutrient and as an energy contribution, because the nitrogen provided by the trees substitutes for the energy required to manufacture and transport an equal amount of urea to the field where it would be applied. Yengwe et al (2018) collected data on the total leaf fall added to the soil surface in maize plots in Zambia and analyzed the nitrogen content over two annual growing seasons This study converts their value for nitrogen addition to an equivalent amount of nitrogen in the form of urea and uses this value as the basis for calculating both the energy input to the system and the climate mitigation benefit (Figures 1, 2). The annual total avoided emissions from the nitrogen provided by F. albida is 0.116 tons CO2 per hectare, as shown in Equation (12)
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