Symbiotic nitrogen fixation in eight Acacia senegal provenances in dryland clays of the Blue Nile Sudan estimated by the super(15)N natural abundance method

The symbiotic biological N sub(2)fixation by Acacia senegal was estimated using the super(15)N natural abundance ( delta super(15)N) procedure on eight provenances collected from different environments and soil types grown in a clay soil in the Blue Nile region, Sudan. Balanites aegyptiaca (a non-legume) was used as a non-N sub(2)-fixing reference plant to allow super(15)N-based estimates of the proportion of the Acacia N derived from atmospheric N sub(2) (N sub(dfa)) to be calculated. Results show variation in leaf delta super(15)N between A. senegal and the reference plant and among years. The relative delta super(15)N values (ppt) were higher in B. aegyptiaca than in the N sub(2)-fixing acacia provenances. Provenances originally collected from clay soils fixed little N in the first year, but the amount fixed increased as the trees aged. All provenances showed a decrease in delta super(15)N with age. The N sub(dfa) varied between 24% (Mazmoom provenance) and 61% (Rahad provenance) 4 years after planting. There was no significant difference in delta super(15)N between provenance groups based on soil type or rainfall at original growing site. The amount of N sub(dfa) increased significantly with age in all provenances. The above-ground contribution of fixed N to foliage growth in a 4-year-old A. senegal was highest in the Rahad sand-soil provenance (46.7 kg N ha super(-1)) and lowest in the Mazmoom clay-soil provenance (28.7 kg N ha super(-1)). Our study represents the first use of the delta super(15)N method for estimating the N input by A. senegal to the clay plain soils of the gum belt in the Sudan.