Limited variation in sorghum yield responses to diverse legume rotations under Sudano-Sahelian conditions

Context In Sudano-Sahelian West Africa, declining soil fertility due to land degradation, low fertilizer use, and shortened fallows threatens productivity. Legume-based rotations offer a sustainable solution by improving nitrogen availability and soil health, especially under low-input conditions. Objectives This study aimed to enhance sorghum-based cropping systems through one-year legume rotations. It sought to characterize a diversity of legume crops and evaluate the effect of returning their biomass (excluding grain) to the soil on subsequent sorghum crop’s growth and yield. Methods Twenty crop precedents, comprising 17 legumes and 3 grasses, were evaluated across three randomized complete block design (RCBD) field trials under varying soil fertility conditions. Biological nitrogen fixation was quantified using the natural abundance (δ¹⁵N) method along with measurements of nitrogen content and total nitrogen accumulation in aboveground biomass. In the following season, sorghum was grown to assess rotational effects on yield and growth. Results The study revealed substantial and significant variability among legume species in biomass and nitrogen accumulation, with Crotalaria juncea and Centrosema pascuorum showing the highest values. However, these differences did not translate into significant yield gains for subsequent sorghum crops. While legumes outperformed grasses in improving sorghum yields, variation among legume species was minimal, suggesting nitrogen recycling inefficiencies under Sudano-Sahelian conditions. Conclusions Under Sudano-Sahelian conditions, non-nitrogen (non-N) effects appear to play a crucial role in the overall rotational benefits of legumes in cereal-based systems. Therefore, grain and fodder legumes should be preferred over green manure legumes because they provide high-protein food for humans and feed for animals, while enhancing the overall performance of crop rotations. Further diversification of legume crops is needed to optimize legume-nonlegume balance and manage trade-offs between food security and sustainable soil management.