This study investigates the impact of bio-char additions on biological nitrogen fixation (BNF) in common beans (Phaseolus vulgaris L.). Bio-char was added at 0, 30, 60, and 90 g kg−1 soil, and BNF was measured using the isotope dilution method after adding 15N-enriched ammonium sulfate. The results showed that BNF increased from 50% without bio-char to 72% with 90 g kg−1 bio-char. Total atmospheric-derived nitrogen (Ndfa) significantly increased with bio-char additions, but decreased at 90 g kg−1 due to low biomass production and N uptake. The primary reason for higher BNF was increased availability of boron (B) and molybdenum (Mo), while other factors like potassium (K), calcium (Ca), and phosphorus (P) availability, pH, and aluminum (Al) saturation also contributed. Enhanced mycorrhizal infections did not significantly improve nutrient uptake or BNF. Bean yield and biomass production increased with bio-char additions at 90 and 60 g kg−1, but decreased at 90 g kg−1 due to reduced biomass production and total N uptake. The study highlights the potential of bio-char to improve N input into agroecosystems, but long-term field studies are needed to fully understand its effects on BNF.This study investigates the impact of bio-char additions on biological nitrogen fixation (BNF) in common beans (Phaseolus vulgaris L.). Bio-char was added at 0, 30, 60, and 90 g kg−1 soil, and BNF was measured using the isotope dilution method after adding 15N-enriched ammonium sulfate. The results showed that BNF increased from 50% without bio-char to 72% with 90 g kg−1 bio-char. Total atmospheric-derived nitrogen (Ndfa) significantly increased with bio-char additions, but decreased at 90 g kg−1 due to low biomass production and N uptake. The primary reason for higher BNF was increased availability of boron (B) and molybdenum (Mo), while other factors like potassium (K), calcium (Ca), and phosphorus (P) availability, pH, and aluminum (Al) saturation also contributed. Enhanced mycorrhizal infections did not significantly improve nutrient uptake or BNF. Bean yield and biomass production increased with bio-char additions at 90 and 60 g kg−1, but decreased at 90 g kg−1 due to reduced biomass production and total N uptake. The study highlights the potential of bio-char to improve N input into agroecosystems, but long-term field studies are needed to fully understand its effects on BNF.