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 of ¹⁵N (δ¹⁵N value) in 11 woody species of the Mimosacease which were compared with the δ¹⁵N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N₂ fixation (N _f ) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida 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 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.