Nitrogen Dynamics of the Acacia Senegal Agroforestry System in the Sudan
The savanna is characterized by low rainfall, poor soil and vegetation types dominated by the Genus Acacia. Agroforestry systems evolved, the most notable of which is Acacia senegal agroforestry system (ASAS). It yields gum acacia, fuel and fodder and improved soil N, P, and pH and stability against erosion. This study investigated the climatic, edaphic and biotic interactions of the ASAS that govern the nutrient dynamics, mainly N, and biomass. The effects of limited resources of soil, space and photoperiod on biomass, nodulation and rate of N fixation were investigated. Genetic differences among populations were studied. Progenies of selected high yielding gum trees were compared. Seed from various localities were grown on different soils to test variation in nodulation potentials. Nodule ultrastructure was also examined. Seedlings raised on clay soil yielded more nodules and biomass than those on sand. Nodule extracts used to infect seedlings raised on sterile soil gave inconsistent nodule numbers. Nodulation was affected by phenological condition. The functional nodule peak period was eight weeks; Extending the period to 13 weeks resulted in less red nodules. Competition intensity correlated negatively with individual plant biomass and nodule numbers. Site potential expressed as total biomass was realized irrespective of plant number. Seed, leaves and twigs contained the highest nitrogen concentration. Iron and phosphorus concentrations were variable among tissues. The bush fallow rotation phase increased soil N, P and pH. Long period of fallow caused more accumulation and better soil improvement. Agricultural crops drastically drained soil N. The marked acidity of Aldubeibat sand was probably due to leaching of basic cations caused by rainfall which was higher than that of Elobeid where sand was at near neutral pH. Acidity impacted nodulation. The variation in soil types and rainfall regime and geographic isolation resulted in genetically different populations. Artificial selection of high yielding mother trees resulted in locating and identifying superior gum trees. They are recommended to be treated as seed stands for dissemination of high quality seed. Low gum producers yielded more biomass than high gum yielders; their effects on nutrient dynamics was insignificant.
Agroforestry, Sudan, Nitrogen, Acacia, Ecology and Evolutionary Biology
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