Greenhouse Gas Emissions, Carbon Footprint, and Grain Yields of Rice-Based Cropping Systems in Eastern China

A multiple cropping system is beneficial for utilizing natural resources, while increasing the grain production and economic outputs. However, its impact on greenhouse gas emissions is unclear. The objective of this study was to evaluate the influence of rice-based cropping systems on methane (CH4) and nitrous oxide (N2O) emissions, the carbon footprint (CF), grain yields, and net economic returns in eastern China. Four treatments were applied: rice-fallow (as a control), rice-milk vetch, rice-wheat, and rice-rapeseed. Methane and N2O emissions were measured every 7 days via static chamber and gas chromatography methods from the 2019 rice season to the 2021 non-rice season. The CF was calculated based on the life cycle assessment. The results showed that multiple cropping systems significantly increased the annual grain yield by 1.2-6.4 t ha(-1) and the annual CH4 and N2O emissions by 38-101 kg CH4-C ha(-1) and 0.58-1.06 kg N2O-N ha(-1), respectively. The average annual net returns for rice-wheat and rice-rapeseed were 131-150% greater than those for rice-milk vetch and rice-fallow. The annual CFs increased in the following order: rice-wheat (19.2 t CO2-eq ha(-1)) > rice-rapeseed (16.6 t CO2-eq ha(-1)) > rice-milk vetch (13.9 t CO2-eq ha(-1)) > rice-fallow (11.5 t CO2-eq ha(-1)). The CH4 emissions contributed to the largest share of the CF (60.4-68.8%), followed by agricultural inputs (27.2-33.7%) and N2O emissions (2.9-5.9%). Moreover, nitrogen fertilizer accounted for 65.6-72.4% of the indirect greenhouse gas emissions from agricultural inputs. No significant difference in the CF per unit grain yield was observed between the four rice-based cropping systems. The CF per net return of rice-wheat and rice-rapeseed significantly decreased by 37-50% relative to that of rice-fallow and rice-milk vetch. These findings suggest the potential to optimize rice-based cropping systems for environmental sustainability and grain security.