Estimates of nitrogen fixation by trees on an aridity gradient in Namibia

Nitrogen (N₂) fixation was estimated along an aridity gradient in Namibia from the natural abundance of15N (δ15N value) in 11 woody species of the Mimosaceae which were compared with the δ15N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N₂ fixation ($\text{N}_{f}$) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida$\text{N}_{f}$was only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species$\text{N}_{f}$was 10-30%. There was a marked variation in background δ15N values along the aridity gradient, with the highest δ15N values in the lowland savanna. The difference between δ15N values of Mimosaceae and non-Mimosaceae, which is assumed to result mainly from N₂ fixation, was also largest in the lowland savanna. Variations in δ15N of Mimosaceae did not affect N concentrations, but higher δ15N-values of Mimosaeae are associated with lower carbon isotope ratios (δ13C value). N₂ fixation was associated with reduced intrinsic water use efficiency. The opposite trends were found in non-Mimosaceae, in which N-concentration increased with δ15N, but δ13C was unaffected. The large variation among species and sites is discussed.