The symbiotic biological N 2 fixation by Acacia senegal was estimated using the 15 N natural abundance (d 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 2 -fixing reference plant to allow 15 N-based estimates of the proportion of the Acacia N derived from atmospheric N 2 (N dfa ) to be calculated. Results show variation in leaf d 15 N between A. senegal and the reference plant and among years. The relative d 15 N values (&) were higher in B. aegyptiaca than in the N 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 d 15 N with age. The N dfa varied between 24% (Mazmoom provenance) and 61% (Rahad provenance) 4 years after planting. There was no significant difference in d 15 N between provenance groups based on soil type or rainfall at original growing site. The amount of N 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 )1 ) and lowest in the Mazmoom clay-soil provenance (28.7 kg N ha )1 ). Our study represents the first use of the d 15 N method for estimating the N input by A. senegal to the clay plain soils of the gum belt in the Sudan.

No abstract

No abstract

No abstract