Submitted to: Global Biogeochemical Cycles
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2002
Publication Date: 3/20/2004
Citation: Li, C., Mosier, A.R., Wassmann, R., Cai, Z., Zheng, X., Huang, Y., Tsuruta, H., Boonjawat, J., Lantin, R. 2004. Modeling greenhouse gas emissions from rice-based production systems: sensitivity and upscaling. Global Biogeochemical Cycles. 18: GB1043, doi:10. 1029/2003GB002045. Interpretive Summary: The atmospheric trace gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are major contributors to global warming and, in historical terms, their concentrations are increasing rapidly. These trace gases are long-lived in the atmosphere and well mixed so that emissions from one part of the world directly influence atmospheric warming throughout the planet. Food production contributes approximately 70 and 40 % of global atmospheric input of N2O and CH4 to the atmosphere, respectively and cropped soils have the potential to sequester atmospheric CO2. Rice-based agriculture is an integral part of global food production and plays a major role in global greenhouse gas exchange. Almost half the people in the world eat rice at least once a day. Rice farms cover 11 percent of the world's arable area. Total rice paddy area in Asia is 1.38 million km2, which accounts for 90% of global rice area (1.55 million km2) and ~20% of total world cropland area for grain production. Rice based crop production is presently undergoing a new revolution. Large scale mechanical tillage, direct seeding rather than transplanting, mid-season draining of rice fields, changing from two rice crops per year to one rice crop and one upland crop, incorporation of rice residue rather than burning, and new rice varieties are among the large changes in rice-based agriculture production management that are either being considered or taking place today across Asia. A demand for predicting effects of the new changes in rice production management in Asia on global atmosphere as well as the local environmental conditions is emerging. To answer the challenge, the process-oriented model DNDC was used for estimating impacts of alternative management measures on the net Global Warming Potential of rice ecosystems at site and regional scales. The study was conducted as a part of an Asian Pacific Network for Global Change Research project on "Land Use/Management Change and Trace Gas Emissions in East Asia".
Technical Abstract: A biogeochemical model, Denitrification-Decomposition (DNDC), was modified to enhance its capacity of predicting greenhouse gas emissions from paddy rice ecosystems. The modifications focused on soil redox potential dynamics, paddy rice management, and rice development and growth. The new model was tested for its sensitivities to the management alternatives commonly practiced in the rice-based production systems. The test results indicated that the most sensitive management practices varied for different greenhouse gases. For example, crop rotation and crop residue incorporation had the most important impacts on the ecosystem net carbon dioxide (CO2) emissions; water management or crop residue management greatly affected methane (CH4) emissions; and drainage of the flooded water substantially increased nitrous oxide (N2O) emissions. In addition, the sensitivity tests also revealed the significant effects of climate and soil conditions on the impacts of management practices. The same management alternative may cause different greenhouse gas emissions from two sites if they differ in climate or soil conditions. Since any single change in management could simultaneously alter the emissions of CO2, CH4, and N2O, the concept of Global Warming Potential (GWP) was adopted to quantify the net warming effect of each management alternative at any specific locations. For the cases tested in the study, that CH4 dominated the rice paddy GWP (60-90% of total GWP) for most of the tested scenarios although N2O dominated when the frequent midseason drainage was applied. The effects of the management alternatives on CO2 accounted for only a small portion (< 15%) of the net GWP values. An upscaling test was conducted to estimate the total warming effect of a specific management alternative, midseason drainage, across climate zones, soil types and land-use management regimes for all of the paddy rice fields in China. The modeled results indicated that total CH4 flux from the simulated 30 million ha of Chinese rice fields, ranged from 6.4-12.0 Tg CH4-C per year under continuous flooding conditions. With the midseason drainage scenario, the national CH4 flux from rice agriculture reduced to 1.7-7.9 Tg CH4-C. It implied the water management change in China reduced CH4 fluxes by 4.7-4.2 Tg CH4-C /yr. Shifting the water management from continuous flooding to midseason drainage increased N2O fluxes by 0.13-0.20 Tg N2O-N/yr although CO2 fluxes were only slightly altered. Since N2O possesses much higher GWP, the increased N2O offset about 65% of the benefit gained by decreasing CH4 fluxes. The conflict between the CH4 and N2O mitigation measures again demonstrates the complexity of mitigating greenhouse gas emissions through managing biogeochemical cycles in terrestrial ecosystems.