Objectives To optimize the fertilization regimes for cropland in the North China Plain, and to maintain crop yields and improve soil fertility while reducing greenhouse gas emissions.

Methods Based on the observation data of long-term experiment, three treatments of NPK, NPKM and OM were used to calibrate and validate the process model (SPACSYS) of modeling crop yields, soil organic carbon (SOC) and total nitrogen (TN) storages, and dynamic changes of soil CO₂ and N₂O emissions, and predict the response of those items to different fertilizers and fertilization scenarios by 2050.

Results Statistical analysis showed that the SPACSYS model gave a reliable and accurate simulation on the wheat yield and maize yields (R² 0.63−0.78, RMSE 3.78%−4.86%, EF 0.59−0.73), SOC and TN storages (R² 0.73−0.89, RMSE 2.69%−3.79%, EF 0.67−0.82) and soil CO₂ and N₂O emissions (R² 0.16−0.80, RMSE 4.03%−9.99%, EF 0.24−0.78), all the three correlations reached significant levels. According to the predictions of the model, reducing 50% of current nitrogen input would significantly reduce maize yield by 9%; reducing 25% of nitrogen fertilizer would not reduce yields significantly, and if combined with manure or applying the same manure N alone, the average annual SOC and TN storages could be significantly improved by 31%, 62% and 18%, 6%, while the average annual CO₂ and N₂O emissions would not be significantly increased, compared with single application of chemical fertilizers.

Conclusions The SPACSYS model can simulate crop yields, SOC and TN storages, and soil CO₂ and N₂O emissions of the winter wheat−summer maize cropping system in the typical farmland of the North China Plain. However, the model underestimates the total N storages when organic manure is applied alone, need further improvements of the model parameters for the situation. According to the prediction, both the application of chemical fertilizers and manures can increase the SOC and TN storages with different degrees, and 25% of reduction of current nitrogen fertilizer level under combined application with manure or application of manure N alone could maintain crop yields, improve soil fertility and reduce greenhouse gas emissions in this region by 2050.