Global use of reactive nitrogen (N) has increased over the past century to meet growing food and biofuel demand, while contributing to substantial environmental impacts. To project future N inputs for crop production, many studies assumed that Nitrogen Use Efficiency (NUE) remains the same as the current level under a Business-As-Usual (BAU) scenario. This assumption ignores potential NUE changes caused by shifting crop mixes and the diminishing return of yield increase to N inputs at a given level of technology and management practices (TMP). To evaluate the impacts of these two factors on the projection of future N inputs, we developed and tested three approaches, namely “Same NUE”, “Same TMP”, and “Improving TMP”. We found that the approach considering the diminishing returns in yield response (“Same TMP”) resulted in 268 Tg N yr-1 of N inputs which were 61 and 48 Tg N yr-1 higher when keeping NUE at the current level with and without considering crop mix, respectively. If TMP is assumed to continue to evolve at the pace of past five decades, the projected N inputs reduce to 204 Tg N yr-1, but still 59 Tg N yr-1 higher than the inputs in the baseline year 2006. Overall, our results suggest that the BAU approach that assumes constant NUE may be too optimistic in projecting N inputs, and the full range of projection assumptions need to be carefully explored when investigating future N budgets.