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Nitrogen (N) fertilization has been repeatedly reported to strongly influence soil properties and crop growth. However, there is little information about the combined effects of soils and crops on soil CO 2 fluxes under N fertilization. In this study, in situ soil respiration, soil physicochemical properties, microbial communities and crop properties were measured for eight years (2008 to 2016) on the Loess Plateau. Five rates of N fertilization were applied to different plots to compare the soil respiration rate and its temperature sensitivity (Q 10). Nitrogen fertilization significantly increased mean annual cumulative soil respiration (R cum) by 25%–44%. R cum had a positive correlation with grain yield, and the carbon emission efficiency (grain yield produced per unit of carbon emission) under N-fertilized plots was 1.62–2.52 times that of unfertilized plots. R cum also had a positive correlation with root biomass and the root N concentration but showed a negative correlation with the root C/N ratio. The Q 10 values under N-fertilized plots decreased by half at a diurnal scale, but had a smaller reduction (i.e. , 0.04–0.09) at an annual scale compared to unfertilized plots. The decreased Q 10 values under N-fertilized plots also resulted from the lower aromaticity of dissolved carbon (SUVA 254) (7.40 vs. 10.53 L mg C−1 m−1). In addition, the altered R cum and Q 10 values were affected by the varied bacteria community derived from N fertilization, which was related to Acidobacteria , Chloroflexi , Proteobacteria and Bacteroidetes. Therefore, N fertilizer applications regulate the combined effects of soil and crop parameters on soil respiration and the Q 10 value. This study suggests that, due to the lower carbon emission efficiency and higher SOC concentration under N-fertilized plots, N fertilizer applications may be used to sustain crop yields and increasing SOC storage while minimizing carbon emission impacts to the environment on the Loess Plateau. • Nitrogen fertilizer application increased soil respiration rate (SR) but decreased Q 10. • Variation in SR and Q 10 was resulted from altered photosynthetic rate, C/N, SUVA 254 , and bacteria community. • Nitrogen fertilizer application increased carbon emission efficiency. [ABSTRACT FROM AUTHOR]