Carbon and water footprints from the production of the major cereal crops in China

Background/Question/Methods Greenhouse gas (GHG) emissions and freshwater scarcity are central environmental concerns in China. China's agriculture contributes to 19% of the national annual GHG emissions and 62% of the water use, and its cereal production accounted for 19.5% of the world in 2011. We believe that there are inextricable tradeoffs or synergies between the carbon footprint (CF) and water footprint (WF) of cereal production, which may also be differentiated among crops and regions. Following the life cycle of cereal production, we developed an integrated model to estimate CF and WF of paddy rice, wheat and corn in China. Furthermore, a field survey was conducted for farmers concerning cropland management from soil preparation to crop harvest in 2011. Based on the survey, we calculated the CF and WF of the three crops and clarified contributions of farming practices to the CF and WF in China.

Results/Conclusions The GHG emissions were 9.9, 3.9, and 3.7 tons of carbon dioxide equivalents per hectare (t CO₂e ha^-1) for rice, wheat, and corn, respectively. Carbon sequestration due to straw return mitigated 202-478 kg CO₂e ha^-1. Therefore, the farm CFs were 9.7, 3.5, and 3.4 t CO₂e ha^-1 for rice, wheat, and corn, respectively. Furthermore, the production CF was 1.4, 0.7, and 0.6 kg CO₂e kg^-1 for rice, wheat, and corn, respectively. The production WF was 1.5, 1.1, and 1 m³ kg^-1 for rice, wheat, and corn, respectively. Blue water made a greater contribution to WF in the north and northwest than to the other regions, and gray water occupied 27% of the total WF. Our results indicate that cereal production in China depends more on fertilization and irrigation than on average global production requirements. The significantly positive relationships between the CF and WF indicate the possibility for simultaneous mitigation. Based on our results and other studies, we supplied potential practices, especially optimized fertilization and irrigation, and straw returns, to mitigate GHG emissions and water use to achieve environmentally sustainable agriculture in China.