Elephant grass has high biomass production potential and can benefit from biological nitrogen fixation (BNF) as its main external nitrogen source. This study evaluated the effect of BNF on biomass productivity and total nitrogen accumulation in different elephant grass genotypes. This experiment was conducted in a 120 m² concrete tank filled with soil labeled with ¹⁵N to estimate the contribution of BNF. The experimental design was randomized blocks with four replications, and the evaluation was over three years of cultivation, with semiannual cuts. The productivity of fresh and dry mass of the shoot, Nitrogen (N) accumulation, and the contribution of BNF by the ¹⁵N natural abundance technique were evaluated. The annual average of BNF was 38%. There was a statistical difference between the treatments, with the genotype P13G13 presenting fresh and dry mass productivity 50% higher than P6G4. The annual average of fresh mass, dry matter, total N, and N derived from BNF in the genotypes was approximately 70, 30, 100 Mg ha⁻¹, and 35 kg ha⁻¹, respectively. The results obtained by the P13G13 genotype allow us to recommend its use for biomass production aimed at bioenergy, favoring sustainability, reducing greenhouse gas emissions, and dependence on synthetic nitrogen fertilizers.