Nitrogen (N) deficiency is a major constraint to agricultural production in Sub-Saharan Africa and biological nitrogen fixation (BNF) is a significant N source in low-input systems. In most studies, estimates of BNF of legumes are based on above ground biomass measurements and underestimate the actual N contribution of legumes. The study was carried out at the FAO/IAEA Laboratories, Seibersdorf, Austria to evaluate BNF in soybean, field pea, pigeon pea and cowpea, and to determine the below ground N contribution using the ¹⁵N stem injection technique. Soybean and field peas derived over 89% of their total N from BNF, but pigeon pea and cowpea did not fix N due to failed inoculation. The BNF-derived N in the soil was greater than the total N in the recovered root fraction. Depending on legume species, 31-66% of the legume N was below ground. On average, 43% of the below ground N for soybean and field pea was derived from the atmosphere. This can be a valuable input in smallholder, low-input agriculture . Measurement of only above-ground and recoverable root fraction N underestimates the amount of N derived from the atmosphere. Therefore, below ground N derived from BNF should be considered when designing cropping systems to optimise the benefits of legumes. The ¹⁵N stem labelling technique is a powerful tool in quantifying below ground N contribution of legumes.