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

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