干旱区生态环境知识资源中心

Nitrogen (N2) fixation was estimated along an aridity gradient in Namibia from the natural abundance of N-15 (delta-N-15 value) in 11 woody species of the Mimosaceae which were compared with the delta-N-15 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 delta-N-15 values along the aridity gradient, with the highest delta-N-15 values in the lowland savanna. The difference between delta-N-15 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-N-15 of Mimosaceae did not affect N concentrations, but higher delta-N-15-values of Mimosaeae are associated with lower carbon isotope ratios (delta-C-13 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-N-15, but delta-C-13 was unaffected. The large variation among species and sites is discussed.