Background and aims Biological nitrogen fixation (BNF) supplies much of soybean ( Glycine max (L.) Merr.) nitrogen (N) demand, but reported fixation rates vary widely, leading to uncertainty in the soybean N cycle. We quantified whole-plant soybean BNF and N allocation using long-term 15 N labeling to assess whether BNF can offset grain N removal in a high-yielding system, and to quantify root contributions to fixed N. Methods...