Contribution of legume nitrogen fixation to sustainable agriculture in Sub-Saharan Africa

Abstract

Grain legumes fix about 15–210 kg N ha −1 seasonally in Africa, and therefore feature prominently in the cropping systems of traditional farmers. However, increased exploitation of this biological N is constrained by various environmental and nutritional factors, including the cropping patterns used. An evaluation of traditional cropping systems in Africa shows that crop rotation involving legume and cereal monocultures is by far more sustainable than intercropping, the most dominant cultural practice in the continent. Tree legumes also fix about 43–581 kg N ha −1 y −1, making their leaf prunings an important component of sustainability in agroforestry and alley cropping systems. In a single year, the prunings of Sesbania sesban can provide up to a hectare of cereal crop, up to 448 kg N, 31.4 kg P, 125 kg K, 114 kg Ca and 27.3 kg Mg, thus making the foliage of this legume the “ideal” fertilizer. Clearly, achieving sustainable yields in Sub-Saharan Africa would require a deeper understanding of how fixed N in legume residues is managed in the soil environment, in addition to expanding the use of neglected African food legumes, which exhibit considerable drought resistance and nitrate tolerance. In Africa, where soil moisture often limits yields, research on neglected, symbiotic native legumes with NO 3 − and drought-tolerant traits would constitute a sound basis for increased sustainable production.