Evaluation of nitrogen and water management on greenhouse gas mitigation in winter wheat-summer maize cropland system in North China

【Objectives】 High-input of ground water and nitrogen (N) fertilizer in cropland maintains crop productivity, while it also increases greenhouse gas (GHG) emission. In this study, we used life-cycle analysis method (from the production and transportation of cropland inputs to crop growth) to accurately evaluate the effect of management practices on the mitigation of GHG emission in agricultural ecosystem. 【Methods】 We analyzed the effect of soil depth on carbon sequestration and the effect of time scale for evaluating mitigation practices on the basis of the proposed evaluation indices, including grain yield and GHG intensity (GHG emission per unit yield). The soil depth of ≥30 cm and the time frame of 20-year were chosen because it isappropriate to reflect changes in soil organic carbon content, in crop yield and N2O emissions as a consequence of management practices. We developed a method that combined the survey data, field measurements and process-based modelling in order to investigate how crop yield and GHG intensity change in the response to management practices for the winter wheat/summer maize rotation at Zhengzhou in North China Plain. Cropland input rates were obtained from common farmer’s practices by questionnaire survey and from published data in the literatures, including irrigation and N application etc. 20-year’s simulation (1990-2010) was conducted with the validated process-based cropping systems model (APSIM) to obtain crop yield, N2O emissions and the change in SOC content in the response to different N application rates (including chemical N and organic N) and irrigation amounts. The simulation results, combined with the indirect CO-eq2 emissions from production and transportation of cropland inputs, were used to evaluate crop productivity and GHG intensity. 【Results】 Compared to the current practices of local farmers, when the annual N application rate is reduced from 520 kg/hm2 to 400 kg/hm2,there is no significant difference in grain yields in 20 years(P=0.39),but the total greenhouse gas emission(AE-GHG) could be reduced by 1.45 t/hm2,emission intensity(GHGI)reduced by 0.08 t/t; When the irrigation amount is reduced from 300 mm to 240 mm,there is no significant difference in grain yield(P=0.39), but the AE-GHG could be reduced by 0.29 t/hm2,(GHGI)by 0.01 t/t. The main reason is the decrease in electricity consumption used for the production and transportation of all the material in agriculture; When all the chemical basal N is replaced by organic materials, there is no significant difference in the grain yield in 20 years’ period (P=0.63),but the AE-GHG could be reduced by 0.03 t/hm2,and GHGI kept constant. The reason is that organic materials could improve the soil C sequestration, but increase the greenhouse gas emission during compost and transportation to field. 【Conclusions】 In North China at moment, the main approach in GHG mitigation strategy should be focused on the optimization of N application and irrigation. When the annual N fertilizer rate is reduced by 120 kg/hm2,irrigation by 60 mm, the approximated GHG emission in 20 years will be reduced by 1.45 and 0.29 t/(hm2·a), respectively.