Modeling Impacts of Farming Management Alternatives on Greenhouse Gas Emissions: A Case Study for Rice Agriculture of China
Since early 1980s, water management for rice agriculture in China has substantially changed, with midseason drainage gradually replacing the traditional practice, continuous flooding over this 20-year period. This change provided an opportunity for us to estimate how a management alternative could impact greenhouse gas emissions at a large regional scale. To answer this question, we developed a tool by integrating a process-based model, DNDC, with a GIS database. With this tool, we simulated carbon sequestration in and CH4 and N2O emissions from all of the rice paddies (about 30 million ha) in China based on 1990 climate and management conditions. Two water management scenarios, continuous flooding and midseason drainage, were applied in the simulations to quantify their effects on emissions of the three greenhouse gases. The modeled results indicated significant impacts of the change in water management on both CH4 and N2O emissions although CO2 fluxes were only slightly altered. Methane emissions were reduced over the 20-year period by about 40 percent, or by 5 Tg/yr, roughly 5-10 percent of total global methane emissions from rice paddies. The mitigating effect of midseason drainage on CH4 was highly uneven across the country with the highest reduction rates (200-290 kg CH4-C/ha) for provinces Hainan, Sichuan, Hubei and Guangdong where double- or triple-cropping rice systems dominated with warm weather and high-clay soils, and the lowest reduction rates (less than 25 kg CH4-C/ha) for Tianjin, Hebei, Ningxia, Liaoning and Gansu where there were only single cropping systems with relatively cool weather and low-clay soils. Shifting water management from continuous flooding to midseason drainage increased N2O emissions from Chinese rice paddies by 0.15 Tg N per year, which offset a large fraction (65 percent) of the greenhouse gas benefit gained by the decrease in CH4 emissions. The midseason drainage-induced N2O fluxes were high (higher than 8.0 kg N/ha) in Jilin, Liaoning, Heilongjiang and Xinjiang where the paddy soils contained relatively high organic matter. In conclusion, the water management alternative (i.e., midseason drainage) has been predicted to have very different effects on mitigation when it is implemented across climatic zones, soil types, or cropping systems. The maximum CH4 reductions and minimum N2O increases can be obtained when the alternative management is applied in the rice areas with warm weather, high soil clay content, and low soil organic matter content. The provinces of Sichuan, Hubei, Hunan, Guangdong, Guangxi, Anhui and Jiangsu, which possess 60 percent of the rice paddies in China and produce 65 percent of China's rice harvest, fall into the category.
Joint Assembly Meeting, American Geophysical Union
American Geophysical Union Publications
Li, C., Frolking, S., Xiao, X., Salas, W., Moore, B., Boles, S., Qui, J., Huang, Y. and Sass, R. (2004), Modeling Impacts of Farming Management Alternatives on Greenhouse Gas Emissions: A Case Study for Rice Agriculture of China, Eos Trans. AGU, 85(17), Jt. Assem. Suppl., Abstract B11A-01.