Nitrogen-15 isotopic dilution method was used to estimate biological nitrogen fixation (BNF) and nitrogen (N) budget of fourteen (14) soybean varieties using maize as reference crop. The experiment was carried out at Sekou in Southern Benin. The amount of N derived from air (Ndfa kg N ha-1) ranged from 51 for variety BRS 261 to 148 for variety Canarana. In a scenario where the soybean shoot dry matter and grains were removed from the field after harvest and only the fallen leaves were incorporated into the soil, the N budget ranged between -91 (Canarana) and -17 kg N ha-1 (BRS 260). When only soybean grain was exported from the fields and fallen leaves and shoot dry matter are incorporated into the soil, the N budget varied from 7 (BRS 261) to 74 kg N ha-1 (BRS Garantia). The study showed that Canarana, TGX 1448 2E and BRS Paraiso soybean varieties fixed the highest amount of N among the 14 varieties. The inclusion of those soybean varieties in cereal-based cropping systems would help reduce N inputs and improve soil and crop productivity in farming systems in Benin.
Key words: 15N enrichment technique, N2 fixation, N balance, soil fertility, Benin.