Journal
BIOLOGY AND FERTILITY OF SOILS
Volume 50, Issue 2, Pages 307-319Publisher
SPRINGER
DOI: 10.1007/s00374-013-0841-3
Keywords
Reference plants; Plant growth; Plant density; N-2 fixation; % Ndfa; Isotope analysis; delta N-15; delta C-13; Soil N uptake; Net N returns
Categories
Funding
- South African Research Chair in Agrochemurgy and Plant Symbioses
- National Research Foundation
- Tshwane University of Technology
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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, N-2 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 a parts per thousand currency signaEuro parts per thousand 0.05) differences in plant dry matter (DM) yield, N concentration and content, delta N-15, 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***), delta N-15 (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 N-2-fixing efficacy of the microsymbionts nodulating Bambara groundnut was low at some locations in South Africa. The data from this study showed delta C-13 values ranging from -28.01 to -26.20 aEuro degrees, which indicates differences in plant water use efficiency on the different fields studied. Furthermore, the positive correlations between delta C-13 and N-fixed (r = 0.15*) and between delta C-13 and N content (r = 0.14*) suggest a functional relationship between water use efficiency and N-2 fixation, just as the positively significant correlations between delta N-15 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 N-2 fixation and photosynthesis. In the same way, the positive correlation between delta C-13 and DM weight (r = 0.14*), or delta C-13 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 N-2 fixation, and photosynthetic activity, just as N-2 fixation in nodules also seems to stimulate leaf photosynthesis.
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