This study investigates how adding bio-char (charcoal, biomass-derived black carbon) affects biological nitrogen fixation (BNF) in common beans (Phaseolus vulgaris L.). Bio-char was added at 0, 30, 60, and 90 g kg⁻¹ soil. BNF was measured using the isotope dilution method after adding ¹⁵N-enriched ammonium sulfate to a Typic Haplustox soil cropped with a nodulating bean variety (CIAT BAT 477) compared to its non-nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without bio-char to 72% with 90 g kg⁻¹ bio-char. Total N derived from the atmosphere (NdfA) increased significantly with 30 and 60 g kg⁻¹ bio-char, but decreased with 90 g kg⁻¹ due to lower biomass production and N uptake. The main reason for increased BNF was greater B and Mo availability, while higher K, Ca, P availability, higher pH, and lower N availability and Al saturation contributed less. Enhanced mycorrhizal infections did not improve nutrient uptake or BNF. Bean yield increased by 46% and biomass production by 39% at 90 and 60 g kg⁻¹ bio-char, respectively. However, biomass production and total N uptake decreased at 90 g kg⁻¹. Soil N uptake by N-fixing beans decreased by 14, 17, and 50% with 30, 60, and 90 g kg⁻¹ bio-char. C/N ratios increased from 16 to 23.7, 28, and 35. Results show bio-char can improve N input into agroecosystems, but long-term field studies are needed to better understand its effects on BNF. Keywords: Biological N fixation, Boron, Charcoal, Molybdenum, Mycorrhiza, ¹⁵N.This study investigates how adding bio-char (charcoal, biomass-derived black carbon) affects biological nitrogen fixation (BNF) in common beans (Phaseolus vulgaris L.). Bio-char was added at 0, 30, 60, and 90 g kg⁻¹ soil. BNF was measured using the isotope dilution method after adding ¹⁵N-enriched ammonium sulfate to a Typic Haplustox soil cropped with a nodulating bean variety (CIAT BAT 477) compared to its non-nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without bio-char to 72% with 90 g kg⁻¹ bio-char. Total N derived from the atmosphere (NdfA) increased significantly with 30 and 60 g kg⁻¹ bio-char, but decreased with 90 g kg⁻¹ due to lower biomass production and N uptake. The main reason for increased BNF was greater B and Mo availability, while higher K, Ca, P availability, higher pH, and lower N availability and Al saturation contributed less. Enhanced mycorrhizal infections did not improve nutrient uptake or BNF. Bean yield increased by 46% and biomass production by 39% at 90 and 60 g kg⁻¹ bio-char, respectively. However, biomass production and total N uptake decreased at 90 g kg⁻¹. Soil N uptake by N-fixing beans decreased by 14, 17, and 50% with 30, 60, and 90 g kg⁻¹ bio-char. C/N ratios increased from 16 to 23.7, 28, and 35. Results show bio-char can improve N input into agroecosystems, but long-term field studies are needed to better understand its effects on BNF. Keywords: Biological N fixation, Boron, Charcoal, Molybdenum, Mycorrhiza, ¹⁵N.