Abstract The role of the legume in the nitrogen (N) cycle was examined in grazed pastures receiving no N fertilizer of both temperate and tropical regions by simulating the fluxes of N through different processes of the cycle. The amounts of legume‐fixed N required to balance the cycle without invoking a drain on soil organic N reserves (i.e. no net N mineralization) was estimated to vary from 38 to 53% of the above‐ground herbage N or from 20 to 31% on a dry matter (DM) basis for tropical pasture systems with a range of pasture utilization of 10–40%. At higher pasture utilization levels of 50‐70%, more typical of intensively grazed temperate pastures, the N input requirement in the absence of fertilizer N would be 57‐67% of the aboveground herbage N or 35‐45% DM. An examination of the role of each contributory process of recycling ( viz. excreta returns, internal cycling or remobilization from senescing tissues, litter decomposition) suggests that variations in the amounts of internally cycled N would have the greatest impact on the requirement for biologically fixed N at low levels of pasture utilization (10‐40%), while at high pasture utilization levels of 70%, variations in the recovery of excreta‐N would have a major effect on the requirement for fixed‐N to balance the cycle. The amounts of biologically fixed N required to sustain a range of herbage DM yields of 3‐22 t DM ha −1 yr −1 would range from 15 to 158 kg N ha −1 yr −1 for tropical pastures. For intensively managed temperate pastures producing 6‐15 t DM ha −1 yr −1 with a N content of 3·5%, a range of fixation of 120‐352 kg N ha −1 yr −1 is required. These simulations indicate how legume contents of 20‐45% of herbage DM could contribute to productive and sustainable (in terms of N) pasture systems of both temperate and tropical regions