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