N2 fixation, carbon accumulation, and plant water relations in soybean (Glycine max L. Merrill) varieties sampled from farmers' fields in South Africa, measured using 15N and 13C natural abundance

In this study, the natural isotopes of N (15N) and C (13C) were used to evaluate symbiotic N nutrition, C accumulation, and plant water-use efficiency in soybean varieties sampled from 37 farmers' fields across the soybean-producing region of South Africa. The data revealed significant differences in all the parameters measured. Shoot dry matter ranged from 17 to 104 g plant−1, δ15N from −1.11 to +5.51‰, percent N derived from fixation from 21 to 96%, and N-fixed varied from 18 to 298 kg N ha−1. The high δ15N and low %Ndfa in some soybean genotypes were due to inhibition of N2 fixation by soil N uptake. Across the board, soybean variety PAN373 contributed the most symbiotic N (298, 242, and 217 kg N ha−1 in fields 3, 2 and 4, respectively, at Parys), followed by LS6164 (with 271 and 245 kg N ha−1 at Endcot field 1 and Devon field 2, respectively), and LS6150 (with 290 kg N ha−1 in field 1 at Parys). C concentration varied from 44 to 50% in soybean shoots, resulting in high shoot C ranging from 8 to 48 g C plant−1. The δ13C values of soybean shoots ranged from −27.3‰ to −21.1‰ in the 37 fields studied, with PAN1453 from Gransvlei field 1 and PAN737 from Parys field 4 exhibiting much greater δ13C values (−21.1‰ and −23.1‰, respectively), and hence increased water-use efficiency. The positive correlation found between N-fixed and dry matter yield (r = 0.70***), N-fixed and C content (r = 0.62***), and N-fixed and C concentration (r = 0.35*) indicates a functional relationship between N2 fixation and photosynthesis. Conventional tillage, as an agronomic tool, decreased water-use efficiency (or δ13C) in soybean plants possibly through alteration in soil structure and soil water retention. Due to a 2.6 °C higher daily maximum temperature in the North West Province than KwaZulu-Natal and Mpumalanga, soybean plants sampled from that province showed better growth, higher dry matter yield, and enhanced symbiotic performance. This argument was supported by a significantly positive correlation found between average daily maximum temperature and both dry matter yield (r = 0.39*), and N-fixed (r = 0.46**). Furthermore, the plants sampled from North West also showed much greater water-use efficiency due to a 127 and 67 mm less rainfall at that province than KwaZulu-Natal and Mpumalanga, respectively.