Effects of increasing fertilization rates on nitric oxide emission and nitrogen use efficiency in low carbon calcareous soil

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 (EFd) 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 0.43 ± 0.04 (N0) to 2.64 ± 0.35 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 EFd values (0.26-0.36%) were observed. The partial factor productivity of applied nitrogen (PFPN) rapidly decreased with increasing fertilization rates; the relationship could be characterized by a rectangular hyperbolic function (P < 0.01). The expected trade-off between EFd and PFPN was not observed. The on-farm PFPN was only 33 ± 3 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.