Soybean nitrogen fixation dynamics in Iowa, USA

•Biological N fixation (BNF) was maximized early in the seed filling period, achieving a maximum rate of 3 kg N ha–1 day–1.•Biomass production was a better predictor of BNF than seed protein or environmental factors.•The BNF rate during seed-fill was driven by crop growth rate (C supply) rather than seed N accumulation rate (N demand).•The N exported in seed exceeded the N derived from BNF, indicating a negative partial N balance for soybean production.