Biological Nitrogen Fixation by Legumes in Nigerian Cropping Systems: Quantification and Enhancement

This study quantifies biological nitrogen fixation by major grain legumes in Nigerian cropping systems and evaluates management strategies for enhancing BNF contribution to system nitrogen budgets. Biological nitrogen fixation by leguminous crops is a free and renewable nitrogen source that can substitute for mineral nitrogen fertiliser and improve soil fertility in rotation sequences. While the principle of BNF is well understood, quantitative data on actual nitrogen fixation rates by cowpea, soybean, and groundnut under Nigerian field conditions, and the factors most affecting fixation, are limited. This study uses N-15 natural abundance and N-15 dilution methods to measure BNF in cowpea, soybean, and groundnut across contrasting soil phosphorus and pH conditions at six sites in Kano, Kwara, Benue, Anambra, Rivers, and Gombe States. Four rhizobium inoculant treatments are compared against uninoculated controls. Soil phosphorus amendment effect on BNF is evaluated in factorial combination with inoculation. Findings reveal that mean nitrogen fixed across all sites and legumes is 68 kilograms per hectare per season, ranging from 24 to 156 kilograms per hectare. Rhizobium inoculation significantly increases BNF by 42 percent on soils with low native Bradyrhizobium populations, particularly in northern sites. Phosphorus amendment increases BNF by 38 percent, confirming the critical role of phosphorus in nodule function. The study contributes original N-15 based BNF quantification for Nigeria and recommends rhizobium inoculant availability improvement and combined inoculant-phosphorus extension messaging for legume farmers.

Keywords: biological nitrogen fixation, legumes, rhizobium inoculant, N-15 dilution, Nigerian cropping systems.