Optimising cobalt and vitamin B12 supplementation: enhancing growth, biological nitrogen fixation, yield, and nutritional quality of common beans in arid regions

Common bean (Phaseolus vulgaris L.) is a globally important legume that contributes to human nutrition through its high protein content and to agroecosystem sustainability via biological nitrogen fixation (BNF). However, productivity is often limited in arid and nutrient-poor soils due to suboptimal nodulation and N fixation. Cobalt serves as a structural component of vitamin B12 and a cofactor for leghemoglobin and nitrogenase, while exogenous B12 May enhance rhizobial activity and nitrogen assimilation, thereby supporting nodulation and growth. Two field trials were conducted in Egypt’s Nubaria region across consecutive seasons, testing six concentrations of Co (0–14 mg L−1) and vitamin B12 (0–18 mg L−1). Parameters measured included nodule number and mass, nitrogenase activity (as determined by the acetylene reduction assay), vegetative growth, seed yield, and seed composition. ANOVA and regression analyses showed significant linear and quadratic responses to both nutrients. Maximum yield increases were 29.9% (Co) and 34.16% (B12) over controls. Regression identified 8.18 mg L−1 Co and 16 mg L−1 B12 as optimal, enhancing protein, carbohydrate, and mineral content without toxicity. These findings demonstrate that targeted Co and B12 supplementation can improve BNF, growth, and yield of P. vulgaris under stress-prone conditions, though broader validation is needed.