Field assessment of symbiotic N2 fixation in wild and cultivated Cyclopia species in the South African fynbos by 15N natural abundance.

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 delta(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 delta(15)N value of the reference plants and the B value of the test Cyclopia species, where B is the delta(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 delta(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.