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 Mimosacease which were compared with the δ15N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N2fixation (Nf) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida Nfwas only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species Nfwas 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 N2fixation, 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). N2fixation 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.
Lehrstuhl Pflanzenökologie, Universität Bayreuth, Box 101251, W-8580, Bayreuth, Federal Republic of Germany.
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Schulze
Lehrstuhl Pflanzenökologie, Universität Bayreuth, Box 101251, W-8580, Bayreuth, Federal Republic of Germany.
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Gebauer
Institut für Botanik und Mikrobiologie, TU München, Arcisstrasse 21, W-8000, München, Federal Republic of Germany.
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Ziegler
Lehrstuhl Botanik II, Universität Würzburg, Mittlerer Dallenbergweg 64, W-8700, Würzburg, Federal Republic of Germany.
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Lange
Acacia; Carbon isotope ratio; Namibia; Nitrogen fixation; Nitrogen isotope ratio