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Application of organic manure combined with synthetic fertilizer can maintain crop yield and improve soil fertility, but the long-term effects of substituting different proportions of synthetic fertilizers with organic manure on N 2 O emission remain unclear. In this study, field experiments and DNDC model simulations were used to study the long-term effects of substituting synthetic fertilizers with organic manure on crop yield and N 2 O emission. The field experiment was conducted at Guanzhong Plain, northern China, under a wheat-maize cropping system. Six treatments were included: no fertilization (CK); synthetic nitrogen (N), phosphorus (P) and potassium (K) fertilizers (NPK); and 25%, 50%, 75% and 100% of the synthetic N substituted by dairy manure (25%M, 50%M, 75%M, and 100%M), respectively. The DNDC model was calibrated using the field data from the NPK treatment from 2014 to 2017 and was validated for the other treatments. The results showed that the DNDC model can successfully simulate the crop yield (e.g. nRMSE < 5%) and annual N 2 O emission (nRMSE < 20%). In addition, a 30-year simulation found that organic manure substitution treatments could maintain wheat yield well, and the yield variation between different years was small. However, relative to the NPK treatment, the maize yields for the first 6 and 7 years were lower under 50%M and 75%M, and under 100%M maize yields were reduced for the first 15 years. The long-term simulation showed that N 2 O emission of fertilized treatment had an increasing trend over time, especially the 75%M treatment where the N 2 O emission was higher than that of NPK treatment after 25 years of fertilization. The annual mean N 2 O emission under different treatments was, in decreasing order, NPK > 25%M > 50%M > 75%M > 100%M > CK. The yield-scale N 2 O emission and emission factor were highest for the NPK treatment. Considering crop yield, yield stability and N 2 O emission, substitution of 25% synthetic fertilizer by organic manure can simultaneously ensure crop productivity and environmental protection under the tested environment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)