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.