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Nitrogen (N) derived from symbiotic fixation of atmospheric N 2  in wild and cultivated populations of  Cyclopia , a woody endemic genus used to make honeybush tea in the Western Cape of South Africa, was quantified by the  15 N natural abundance method. Because  Cyclopia  species are naturally mycorrhizal, non-N 2 -fixing arbuscular mycorrhizal shrubs of similar phenology to  Cyclopia  were chosen as reference plants to provide the δ 15 N value of soil-derived N. Isotopic analysis showed that wild populations of  Cyclopia  were highly dependent on N 2  fixation for their N nutrition, ranging from 70 ± 4% to 100 ± 7% (mean ± SE) at all sites, except for one. Further evidence of the high dependency of wild  Cyclopia  populations on symbiotic N was provided by their significantly higher foliar N concentrations compared with the non-legume reference plants. However, cultivated  Cyclopia  exhibited variable amounts of N 2  fixation, with  Cyclopia genistoides  (L.) R. Br., for example, showing low amounts of N 2  fixation at Sites P2 and P3 (0 ± 51% and 8 ± 46%, respectively) as a result of low  D  values ( D  is defined as the difference between the mean δ 15 N value of the reference plants and the  B  value of the test  Cyclopia  species, where  B  is the δ 15 N of an inoculated test legume grown in an N-free growth medium), whereas at Sites P1, P2, P5 and P6, up to 89 ± 2%, 94 ± 13%, 85 ± 13% and 100 ± 18%, respectively, plant N was derived from atmospheric fixation. The high symbiotic N nutrition observed for wild populations of  Cyclopia  suggests that these populations are major contributors to the N economy of the nutrient-poor soils of the South African fynbos. These data indicate that by breeding for high N 2  fixation rates in  Cyclopia  cultivars and selecting more efficient rhizobial strains, this legume has the potential to achieve higher N 2  fixation rates under cultivation. The low variability in  Cyclopia  δ 15 N values within sites, however, suggests that genetic variability is not a major factor influencing N 2  fixation rates in cultivated  Cyclopia , and that more benefit may be gained from soil amelioration and the selection of improved rhizobial strains.

Field assessment of symbiotic N 2 fixation in wild and cultivated Cyclopia species in the South African fynbos by 15 N natural abundance