Symbiotic N nutrition, C assimilation, and plant water use efficiency in Bambara groundnut (Vigna subterranea L. Verdc) grown in farmers’ fields in South Africa, measured using ¹⁵N and ¹³C natural abundance

Bambara groundnut (Vigna subterranea L. Verdc) is the second most important indigenous food legume in Africa. The aim of this study was to evaluate plant growth, N₂ fixation, N contribution, C accumulation, and plant water relations of Bambara groundnut grown in 26 farmers’ fields in Mpumalanga Province of South Africa. The data revealed marked (p ≤ 0.05) differences in plant dry matter (DM) yield, N concentration and content, δ¹⁵N, the proportion of N derived from symbiotic fixation (%Ndfa), and actual amounts of N-fixed between and among the 26 farms surveyed. Bambara groundnut plants obtained 33–98 % (mean = 72 %) of their N nutrition from symbiotic fixation and contributed 4–200 kg N-fixed ha⁻¹ (mean = 102 kg N-fixed ha⁻¹). Plant density correlated positively with %N (r = 0.31***), δ¹⁵N (r = 0.126***), and amount of N-fixed (r = 0.15*), indicating that the high %Ndfa values obtained for Bambara groundnut in this study and the low symbiotic N yield associated with some farms were due to low plant density rather than poor symbiotic functioning. Bambara groundnut obtained more N from soil (e.g., 173 kg N ha⁻¹) than from symbiosis (e.g., 135 kg N-fixed ha⁻¹) in some fields, implying that the N₂-fixing efficacy of the microsymbionts nodulating Bambara groundnut was low at some locations in South Africa. The data from this study showed δ¹³C values ranging from −28.01 to −26.20 ‰, which indicates differences in plant water use efficiency on the different fields studied. Furthermore, the positive correlations between δ¹³C and N-fixed (r = 0.15*) and between δ¹³C and N content (r = 0.14*) suggest a functional relationship between water use efficiency and N₂ fixation, just as the positively significant correlations between δ¹⁵N and DM yield (r = 0.24***), N-fixed and DM weight (r = 0.76**), and N content and DM yield (r = 0.99*), as well as N-fixed and C content (r = 0.76**) also indicate a functional relationship between N₂ fixation and photosynthesis. In the same way, the positive correlation between δ¹³C and DM weight (r = 0.14*), or δ¹³C and C content (r = 0.15*), also implies a functional link between water use efficiency and plant growth. Thus, an increase in water use efficiency in Bambara groundnut, whenever it occurs, seems to functionally enhance plant growth, symbiotic N₂ fixation, and photosynthetic activity, just as N₂ fixation in nodules also seems to stimulate leaf photosynthesis.