Understanding nitrous oxide (N2O) fluxes from grain–legume crops in semiarid and arid regions is necessary if we are to improve our knowledge of global terrestrial N2O losses resulting from biological N2 fixation. N2O fluxes were measured from a rain-fed soil, cropped to a grain–legume in a semiarid region of southwestern Australia for 1 year on a subdaily basis. The site included plots planted to narrow-leafed lupin (Lupinus angustifolius; ‘lupin’) and plots left bare (no lupin). Fluxes were measured using soil chambers connected to a fully automated system that measured N2O by gas chromatography. Daily N2O fluxes were low (−0.5 to 24 g N2O-N ha−1 day−1) and not different between treatments, culminating in an annual loss of 127 g N2O-N ha−1. Greatest daily N2O fluxes occurred from both treatments in the postharvest period, and following a series of summer and autumn rainfall events. At this time of the year, soil conditions were conducive to soil microbial N2O production: elevated soil water contents, increased inorganic nitrogen (N) and dissolved organic carbon concentrations, and soil temperatures generally > 25 °C; furthermore, there was no active plant growth to compete for mineralized N. N2O emissions from the decomposition of legume crop residue were low, and approximately half that predicted using the currently recommended IPCC methodology. Furthermore, the contribution of the biological N2 fixation process to N2O emissions appeared negligible in the present study, supporting its omission as a source of N2O from the IPCC methodology for preparing national greenhouse gas inventories.