| Schulze, ED; Gebauer, G; Ziegler, H; Lange, OL: Estimates of nitrogen fixation by trees on an aridity gradient in Namibia, Oecologia, 88, 451-455 (1991), doi:10.1007/BF00317592 | |
| Abstract: Nitrogen (N2) fixation was estimated along an aridity gradient in Namibia from the natural abundance of 15N (delta-15N value) in 11 woody species of the Mimosaceae which were compared with the delta-15N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N2 fixation (Nf) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida Nf was only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species Nf was 10-30%. There was a marked variation in background delta-15N values along the aridity gradient, with the highest delta-15N values in the lowland savanna. The difference between delta-15N values of Mimosaceae and non-Mimosaceae, which is assumed to result mainly from N2 fixation, was also largest in the lowland savanna. Variations in delta-15N of Mimosaceae did not affect N concentrations, but higher delta-15N-values of Mimosaeae are associated with lower carbon isotope ratios (delta-13C 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 delta-15N, but delta-13C was unaffected. The large variation among species and sites is discussed. |
