Effects of substituting synthetic nitrogen with organic amendments on crop yield, net greenhouse gas emissions and carbon footprint: A global meta-analysis

Excessive application of synthetic nitrogen fertilizer and discharge of organic wastes were the primary sources of greenhouse gas (GHG) emissions in agricultural system. Synthetic nitrogen fertilizer substitution (NSS) practice by organic amendments was considered as a possible strategy to decrease the on-field N2O emission and simultaneously increase the soil organic carbon (SOC) and yield in cropping system. The effects of NSS practice on the trade-offs between crop yield and net greenhouse (NGHG) emissions have been still uncertain at a global level at present. This study carried out a joint use of meta-analysis, NGHG balance assessment and carbon footprint (CF) to quantitatively assess the effects of the NSS practices from multi-points. Results showed the NSS practice improved crop yield by 2.00 %, showing a positive effect at a global level, especially in rice and maize production systems. Meanwhile, the NSS practice significantly increased SOC storage by 23.97 %. For the on-field N2O and CH4 emissions, the effect sizes varied large under different factors, without significant difference. The general trends were that the NSS practice decreased the average effect sizes of N2O emission and increased the average effect sizes of CH4 emission. Standing at a farmland and system levels, the NSS practice increased the NGHGs and CFs both at the area and yield scales for paddy system. For upland cropping system, both of the yield-based and area-based CFs were reduced under the NSS practice due to the improved yields of wheat and maize. In conclusion, the NSS practice showed positive effects on improving crop yield and SOC storage at the global level but negative effect on decreasing the on-field GHG emissions. Crop type was the primary influencing factor. According to the current results, we assumed that the NSS would be an effective practice for upland cropping systems because it would generate the less NGHG emission and CF when producing unit mass of grain products. However, the NSS would not be a good practice for the rice production system. The results provided valuable information for the development of sustainable agricultural production in the world.