Global nitrogen fertilizer consumption is expected to continue to increase. To explore effective mitigation strategies, a deeper understanding of the responses of nitrogen use efficiency, nitric oxide (NO) emission and the NO direct emission factor (EF d) to increasing fertilization rates is needed. A gradient of fertilization rates (0, 135, 270, 430, 650 and 850 kg N ha -1 yr -1 in the form of urea, hereafter referred to as N0, N135, N270, N430, N650 and N850, respectively) was used to fully represent the nitrogen application levels in the wheat-maize rotational cropping system that has been widely adopted in China. The annual NO emissions ranged from 0.430.04 (N0) to 2.640.35 kg N ha -1 yr -1 (N850) and linearly increased with increasing fertilization rates ( P<0.01). The high pH and low carbon availability in the calcareous soil limited NO production; thus, low EF d values (0.26-0.36%) were observed. The partial factor productivity of applied nitrogen (PFP N) rapidly decreased with increasing fertilization rates; the relationship could be characterized by a rectangular hyperbolic function ( P<0.01). The expected trade-off between EF d and PFP N was not observed. The on-farm PFP N was only 333 kg grain kg -1 N (N430), highlighting the necessity of optimizing current management strategies. Based on a review of previous studies, a comprehensive optimized management strategy is recommended to obtain the maximum benefits for multiple goals of a wheat-maize cropping system. However, consecutive field observations and model studies are still needed to validate the long-term effects of this management strategy.
